Poly(3,4-ethylenedioxy thiophene) (PEDT), obtained by chemical vapor phase polymerization can shrink to 5% of its original thickness during a washing step. This phenomenon was exploited to immobilize the active enzyme, horseradish peroxidase (HRP), into PEDT films by rinsing with an ethanol solution containing HRP. The use of the PEDT-HRP as an amperometric biosensor was demonstrated by sensing 5 mM H2O2 in phosphate buffered saline solution with a sensitivity of 190 mu A cm(-2). The stability of the immobilization of HRP into the ''stuffed'' PEDT matrix was also characterized by repeated amperometric detection and scanning electron microscopy mapping images obtained before and after exposure to H2O2. (c) 2006 The Electrochemical Society.