diff --git a/src/ser.rs b/src/ser.rs
index b4737cc..90cfcd7 100644
--- a/src/ser.rs
+++ b/src/ser.rs
@@ -569,8 +569,9 @@ impl<'a> Serializer<'a> {
                     match ch {
                         '\t' => {}
                         '\n' => ty = Type::NewlineTripple,
-                        // note that the following are invalid: \b \f \r
-                        c if c < '\u{1f}' => can_be_pretty = false, // Invalid control character
+                        // Escape codes are needed if any ascii control
+                        // characters are present, including \b \f \r.
+                        c if c <= '\u{1f}' || c == '\u{7f}' => can_be_pretty = false,
                         _ => {}
                     }
                     out.push(ch);
@@ -646,7 +647,7 @@ impl<'a> Serializer<'a> {
                         '\u{d}' => self.dst.push_str("\\r"),
                         '\u{22}' => self.dst.push_str("\\\""),
                         '\u{5c}' => self.dst.push_str("\\\\"),
-                        c if c < '\u{1f}' => {
+                        c if c <= '\u{1f}' || c == '\u{7f}' => {
                             write!(self.dst, "\\u{:04X}", ch as u32).map_err(ser::Error::custom)?;
                         }
                         ch => self.dst.push(ch),
diff --git a/test-suite/tests/valid/string-escapes.json b/test-suite/tests/valid/string-escapes.json
index 62dac51..e3b2d94 100644
--- a/test-suite/tests/valid/string-escapes.json
+++ b/test-suite/tests/valid/string-escapes.json
@@ -46,5 +46,13 @@
     "notunicode4": {
         "type": "string",
         "value": "This string does not have a unicode \\\u0075 escape."
+    },
+    "delete": {
+        "type": "string",
+        "value": "This string has a \u007f delete control code."
+    },
+    "unitseparator": {
+        "type": "string",
+        "value": "This string has a \u001f unit separator control code."
     }
 }
diff --git a/test-suite/tests/valid/string-escapes.toml b/test-suite/tests/valid/string-escapes.toml
index c5d4954..152c6ad 100644
--- a/test-suite/tests/valid/string-escapes.toml
+++ b/test-suite/tests/valid/string-escapes.toml
@@ -10,3 +10,5 @@ notunicode1 = "This string does not have a unicode \\u escape."
 notunicode2 = "This string does not have a unicode \u005Cu escape."
 notunicode3 = "This string does not have a unicode \\u0075 escape."
 notunicode4 = "This string does not have a unicode \\\u0075 escape."
+delete = "This string has a \u007F delete control code."
+unitseparator = "This string has a \u001F unit separator control code."