diff --git a/libraries/stdlib/src/kotlin/io/Constants.kt b/libraries/stdlib/src/kotlin/io/Constants.kt
index 9838589ef1b..2832a0fca2b 100644
--- a/libraries/stdlib/src/kotlin/io/Constants.kt
+++ b/libraries/stdlib/src/kotlin/io/Constants.kt
@@ -7,11 +7,14 @@ package kotlin.io
  * Returns the default buffer size when working with buffered streams.
 */
 public val defaultBufferSize: Int = 64 * 1024
+
 /**
  * Returns the default block size for forEachBlock().
  */
+@Deprecated("This constant will become private soon, use its value instead.", ReplaceWith("4096"))
 public val defaultBlockSize: Int = 4096
 /**
  * Returns the minimum block size for forEachBlock().
  */
+@Deprecated("This constant will become private soon, use its value instead.", ReplaceWith("512"))
 public val minimumBlockSize: Int = 512
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