diff --git a/R/AddStep.R b/R/AddStep.R
index 037bd58ca9aae1f11be3042dd046dc488d9ac005..a129f1ef80260a7969ed3cb5f220b1f4b040f31b 100644
--- a/R/AddStep.R
+++ b/R/AddStep.R
@@ -44,19 +44,18 @@
 #'@export
 AddStep <- function(inputs, step_fun, ...) {
   # Check step_fun
-  if (!('startR_step_fun' %in% class(step_fun))) {
+  if (!is(step_fun, 'startR_step_fun')) {
     stop("Parameter 'step_fun' must be a startR step function as returned by Step.")
   }
   
   # Check inputs
-  if (any(c('startR_cube', 'startR_workflow') %in% class(inputs))) {
+  if (is(inputs, 'startR_cube') | is(inputs, 'startR_workflow')) {
     inputs <- list(inputs)
     names(inputs) <- 'input1'
   }
   else if (is.list(inputs)) {
     if (any(!sapply(inputs, 
-                    function(x) any(c('startR_cube', 
-                                      'startR_workflow') %in% class(x))))) {
+                    function(x) is(x, 'startR_cube') | is(x, 'startR_workflow')))) {
       stop("Parameter 'inputs' must be one or a list of objects of the class ",
            "'startR_cube' or 'startR_workflow'.")
     }
@@ -90,7 +89,7 @@ AddStep <- function(inputs, step_fun, ...) {
       stop("The target dimensions required by 'step_fun' for the input ", input, 
            " are not present in the corresponding provided object in 'inputs'.")
     }
-    if ('startR_workflow' %in% class(inputs[[input]])) {
+    if (is(inputs[[input]], 'startR_workflow')) {
       if (is.null(previous_target_dims)) {
         previous_target_dims <- attr(inputs[[input]], 'TargetDims')
       } else {
diff --git a/R/ByChunks.R b/R/ByChunks.R
index 8185763bf5c31d388bdb28d5ab3873d8636f46d9..4782ef275557a1988cbd8d21db6d67b2c8d2e1df 100644
--- a/R/ByChunks.R
+++ b/R/ByChunks.R
@@ -109,7 +109,7 @@ ByChunks <- function(step_fun, cube_headers, ..., chunks = 'auto',
   MergeArrays <- .MergeArrays
   
   # Check input headers
-  if ('startR_cube' %in% class(cube_headers)) {
+  if (is(cube_headers, 'startR_cube')) {
     cube_headers <- list(cube_headers)
   }
   if (!all(sapply(lapply(cube_headers, class), 
@@ -411,7 +411,7 @@ ByChunks <- function(step_fun, cube_headers, ..., chunks = 'auto',
   timings[['nchunks']] <- prod(unlist(chunks))
   
   # Check step_fun
-  if (!('startR_step_fun' %in% class(step_fun))) {
+  if (!is(step_fun, 'startR_step_fun')) {
     stop("Parameter 'step_fun' must be of the class 'startR_step_fun', as returned ",
          "by the function Step.")
   }
diff --git a/R/Collect.R b/R/Collect.R
index bf387297722c15e61e36d6b768624aec0b73c605..4c80b037d4b91101fc0e90fc6c5c3b1f09b5d624 100644
--- a/R/Collect.R
+++ b/R/Collect.R
@@ -72,7 +72,7 @@
 #'
 #'@export
 Collect <- function(startr_exec, wait = TRUE, remove = TRUE) {
-  if (!('startR_exec' %in% class(startr_exec))) {
+  if (!is(startr_exec, 'startR_exec')) {
     stop("Parameter 'startr_exec' must be an object of the class ",
          "'startR_exec', as returned by Collect(..., wait = FALSE).")
   }
diff --git a/R/Compute.R b/R/Compute.R
index 0e8d42cd134a3bbc66fc896a357db1e4f5cd65a5..778d9ced74508a59a1e5512e8d4d93bb2cab45df 100644
--- a/R/Compute.R
+++ b/R/Compute.R
@@ -89,13 +89,13 @@ Compute <- function(workflow, chunks = 'auto',
                     ecflow_server = NULL, silent = FALSE, debug = FALSE,
                     wait = TRUE) {
   # Check workflow
-  if (!any(c('startR_cube', 'startR_workflow') %in% class(workflow))) {
+  if (!is(workflow, 'startR_cube') & !is(workflow, 'startR_workflow')) {
     stop("Parameter 'workflow' must be an object of class 'startR_cube' as ",
          "returned by Start or of class 'startR_workflow' as returned by ",
          "AddStep.")
   }
   
-  if ('startR_cube' %in% class(workflow)) {
+  if (is(workflow, 'startR_cube')) {
     #machine_free_ram <- 1000000000
     #max_ram_ratio <- 0.5
     #data_size <- prod(c(attr(workflow, 'Dimensions'), 8))
diff --git a/R/SelectorChecker.R b/R/SelectorChecker.R
index 81ec4889ff344b808e1b9ab0569a05d893e80aa6..76899c13450b751dc7ef411f6ff11065de0a29a3 100644
--- a/R/SelectorChecker.R
+++ b/R/SelectorChecker.R
@@ -93,7 +93,7 @@ SelectorChecker <- function(selectors, var = NULL, return_indices = TRUE,
             
             tol <- 0
             if (!is.null(tolerance)) {
-              if (!any(class(tolerance) %in% "numeric")) {
+              if (!is(tolerance, "numeric")) {
                 stop("Expected a numeric *_tolerance.")
               }
               tol <- tolerance
@@ -148,7 +148,7 @@ SelectorChecker <- function(selectors, var = NULL, return_indices = TRUE,
             val <- selectors[[i]]
             tol <- 0
             if (!is.null(tolerance)) {
-              if (!any(class(tolerance) %in% "difftime")) {
+              if (!is(tolerance, "difftime")) {
                 stop("Expected a difftime *_tolerance.")
               }
               tol <- tolerance
@@ -194,7 +194,7 @@ SelectorChecker <- function(selectors, var = NULL, return_indices = TRUE,
                             "nearest values."))
           }
           if (!is.null(tolerance)) {
-            if (!any(class(tolerance) %in% 'numeric')) {
+            if (!is(tolerance, 'numeric')) {
               stop("Expected a numeric *_tolerance.")
             }
           }
@@ -228,7 +228,7 @@ SelectorChecker <- function(selectors, var = NULL, return_indices = TRUE,
                         "nearest values."))
       }
       if (!is.null(tolerance)) {
-        if (!any(class(tolerance) %in% 'difftime')) {
+        if (!is(tolerance, 'difftime')) {
           stop("Expected a difftime *_tolerance.")
         }
       }
diff --git a/R/Start.R b/R/Start.R
index 6fc05ea2d3d8d8c46437adf229040123db1e0f24..f61533603f3ad4f01f1a18ed2361bd7ad0a54ba3 100644
--- a/R/Start.R
+++ b/R/Start.R
@@ -1384,7 +1384,7 @@ Start <- function(..., # dim = indices/selectors,
       # names as depended dim.
       for (j in 1:length(dat_selectors[[file_dim]])) {
         sv <- selector_vector <- dat_selectors[[file_dim]][[j]]
-        if (!identical(first_class, class(sv)) ||
+        if (!is(sv, first_class) ||
             !identical(first_length, length(sv))) {
           stop("All provided selectors for depending dimensions must ",
                "be vectors of the same length and of the same class.")
diff --git a/R/Utils.R b/R/Utils.R
index d0e850e7f8c01180ef043ca5d4b5886ea3abca61..425336eff1b4d908a3030cca40bcbfed65d2ee65 100644
--- a/R/Utils.R
+++ b/R/Utils.R
@@ -806,7 +806,7 @@
               readRDS(paste0(shared_dir, '/',
                              chunk_files_original[found_chunk]))
             })
-            if (('try-error' %in% class(array_of_chunks[[i]]))) {
+            if (is(array_of_chunks[[i]], 'try-error')) {
               message("Waiting for an incomplete file transfer...")
               Sys.sleep(5)
             } else {
diff --git a/R/zzz.R b/R/zzz.R
index 0067d6d88022615a6c8583fe7d22e79e367aeade..83e2b7b7846e6e7427de58d00477f5152884f107 100644
--- a/R/zzz.R
+++ b/R/zzz.R
@@ -1059,7 +1059,7 @@ remove_additional_na_from_merge <- function(data_array = NULL, merge_dim_metadat
     # data_array can be data or metadata; if data, change the blank spaces from 
     # NA to -9999; if metadata (supposed to be 'time'), change the corresponding
     # spaces to -12^10.
-    if (any(class(data_array) %in% c("POSIXct", "POSIXt"))) {
+    if (is(data_array, "POSIXct")) {
       # change to numeric first
       data_array <- array(as.vector(data_array), dim = dim(data_array))
       data_array[which(!logi_array)] <- -12^10