diff --git a/src/Init/Data/Char/Basic.lean b/src/Init/Data/Char/Basic.lean
index f163ef8ed7..58a4de6774 100644
--- a/src/Init/Data/Char/Basic.lean
+++ b/src/Init/Data/Char/Basic.lean
@@ -9,8 +9,6 @@ import Init.Data.UInt
 @[inline, reducible] def isValidChar (n : UInt32) : Prop :=
   n < 0xd800 ∨ (0xdfff < n ∧ n < 0x110000)
 
-instance : SizeOf Char := ⟨fun c => c.val.toNat⟩
-
 namespace Char
 
 protected def Less (a b : Char) : Prop := a.val < b.val
diff --git a/src/Init/Data/String/Basic.lean b/src/Init/Data/String/Basic.lean
index cb05a6e7a0..0b9651c4a2 100644
--- a/src/Init/Data/String/Basic.lean
+++ b/src/Init/Data/String/Basic.lean
@@ -153,8 +153,6 @@ def splitOn (s : String) (sep : String := " ") : List String :=
 
 instance : Inhabited String := ⟨""⟩
 
-instance : SizeOf String := ⟨String.length⟩
-
 instance : Append String := ⟨String.append⟩
 
 def str : String → Char → String := push