diff --git a/specification/trace/sdk.md b/specification/trace/sdk.md
index bbb8194eea5..2454b814016 100644
--- a/specification/trace/sdk.md
+++ b/specification/trace/sdk.md
@@ -491,6 +491,8 @@ exceptions.
   SHOULD be reflected in it.
   For example, this is useful for creating a SpanProcessor that periodically
   evaluates/prints information about all active span from a background thread.
+  When a reference is kept, the span MUST be treated as a readonly object,
+  trying to modify the Span after `OnStart` method returns is undefined behavior.
 * `parentContext` - the parent `Context` of the span that the SDK determined
   (the explicitly passed `Context`, the current `Context` or an empty `Context`
   if that was explicitly requested).