diff --git a/src/env-inl.h b/src/env-inl.h
index cc9dd9ec3e2082..a3f4a797adaeae 100644
--- a/src/env-inl.h
+++ b/src/env-inl.h
@@ -36,12 +36,26 @@ inline void Environment::IsolateData::Put() {
   }
 }
 
+// Create string properties as internalized one byte strings.
+//
+// Internalized because it makes property lookups a little faster and because
+// the string is created in the old space straight away.  It's going to end up
+// in the old space sooner or later anyway but now it doesn't go through
+// v8::Eternal's new space handling first.
+//
+// One byte because our strings are ASCII and we can safely skip V8's UTF-8
+// decoding step.  It's a one-time cost, but why pay it when you don't have to?
 inline Environment::IsolateData::IsolateData(v8::Isolate* isolate,
                                              uv_loop_t* loop)
     : event_loop_(loop),
       isolate_(isolate),
 #define V(PropertyName, StringValue)                                          \
-    PropertyName ## _(isolate, FIXED_ONE_BYTE_STRING(isolate, StringValue)),
+    PropertyName ## _(isolate,                                                \
+                      v8::String::NewFromOneByte(                             \
+                          isolate,                                            \
+                          reinterpret_cast<const uint8_t*>(StringValue),      \
+                          v8::NewStringType::kInternalized,                   \
+                          sizeof(StringValue) - 1).ToLocalChecked()),
     PER_ISOLATE_STRING_PROPERTIES(V)
 #undef V
     ref_count_(0) {}
@@ -464,7 +478,10 @@ inline void Environment::SetMethod(v8::Local<v8::Object> that,
                                    v8::FunctionCallback callback) {
   v8::Local<v8::Function> function =
       NewFunctionTemplate(callback)->GetFunction();
-  v8::Local<v8::String> name_string = v8::String::NewFromUtf8(isolate(), name);
+  // kInternalized strings are created in the old space.
+  const v8::NewStringType type = v8::NewStringType::kInternalized;
+  v8::Local<v8::String> name_string =
+      v8::String::NewFromUtf8(isolate(), name, type).ToLocalChecked();
   that->Set(name_string, function);
   function->SetName(name_string);  // NODE_SET_METHOD() compatibility.
 }
@@ -475,7 +492,10 @@ inline void Environment::SetProtoMethod(v8::Local<v8::FunctionTemplate> that,
   v8::Local<v8::Signature> signature = v8::Signature::New(isolate(), that);
   v8::Local<v8::Function> function =
       NewFunctionTemplate(callback, signature)->GetFunction();
-  v8::Local<v8::String> name_string = v8::String::NewFromUtf8(isolate(), name);
+  // kInternalized strings are created in the old space.
+  const v8::NewStringType type = v8::NewStringType::kInternalized;
+  v8::Local<v8::String> name_string =
+      v8::String::NewFromUtf8(isolate(), name, type).ToLocalChecked();
   that->PrototypeTemplate()->Set(name_string, function);
   function->SetName(name_string);  // NODE_SET_PROTOTYPE_METHOD() compatibility.
 }
@@ -485,7 +505,10 @@ inline void Environment::SetTemplateMethod(v8::Local<v8::FunctionTemplate> that,
                                            v8::FunctionCallback callback) {
   v8::Local<v8::Function> function =
       NewFunctionTemplate(callback)->GetFunction();
-  v8::Local<v8::String> name_string = v8::String::NewFromUtf8(isolate(), name);
+  // kInternalized strings are created in the old space.
+  const v8::NewStringType type = v8::NewStringType::kInternalized;
+  v8::Local<v8::String> name_string =
+      v8::String::NewFromUtf8(isolate(), name, type).ToLocalChecked();
   that->Set(name_string, function);
   function->SetName(name_string);  // NODE_SET_METHOD() compatibility.
 }
diff --git a/src/env.h b/src/env.h
index 1107dcb174bd35..fbbcaf2a8a2720 100644
--- a/src/env.h
+++ b/src/env.h
@@ -39,7 +39,7 @@ namespace node {
 #endif
 
 // Strings are per-isolate primitives but Environment proxies them
-// for the sake of convenience.
+// for the sake of convenience.  Strings should be ASCII-only.
 #define PER_ISOLATE_STRING_PROPERTIES(V)                                      \
   V(address_string, "address")                                                \
   V(args_string, "args")                                                      \