Here we show that the graphene oxide (GO) can be electrochemically reduced in composite films of poly(3,4-ethylenedioxythiophene) (PEDOT) and GO. EDOT was electropolymerized in an aqueous GO dispersion at a constant potential resulting in the incorporation of GO in the PEDOT matrix. Scanning electron microscopy (SEM) images revealed that the formed PEDOT-GO films had a layered structure. X-ray photoelectron spectra measured after 10, 20 and 30 min of electrochemical reduction at -0.85 V, verified that the reduction efficiently removed the epoxy and hydroxyl groups from the GO surface. The number of oxygen-containing functional groups decreased considerably already after 10 min of electrochemical reduction and the C:0 ratio of the composite films increased with increasing reduction time confirming that GO was successfully reduced in the polymer matrix. In contrast to chemical reduction in 0.15 M NaBH4, we show that the PEDOT matrix withstands the electrochemical reduction without any degradation in electroactivity. We also studied the effect of pH of the GO dispersion on the subsequent redox behavior of the PEDOT-GO films. Increasing the pH from 2.5 to 4.5 improved the electroactivity of the films and also facilitated film formation probably due to the presence of a higher amount of ionized carboxylic groups on the GO surface. Electrochemical impedance measurements showed that increasing the pH of the GO dispersion resulted in films with a higher redox capacitance. Atomic force microscopy measurements revealed that the electrochemical reduction slightly increased the surface roughness of the composite films. The simple and fully electrochemical synthesis and reduction procedure of the PEDOT-GO films are expected to be useful in the fabrication of interfacial materials for electrochemical all-solid-state devices where it is desirable to have reversible ion-to-electron transduction in combination with high redox capacitance. (C) 2013 Elsevier Ltd. All rights reserved.