Federal Reserve Chairman Ben Bernanke says the end of the central bank’s bond buying won’t constitute a move toward tighter policy. He may have a tough time convincing stock and bond investors that’s true.

The Fed is acquiring $85 billion of securities each month, and policy makers are grappling with how to condition markets not to interpret a stop in those purchases as a prelude to the exit from easy credit. Bernanke said Dec. 12 in Washington that he “would emphasize” the end won’t be “a turn to tighter policy.”