The potentiostatic deposition of polypyrrole (PPy) from 0.1 M aqueous phosphoric acid solution and the doping/dedoping behavior of the resulting films have been investigated by the electrochemical quartz crystal microbalance (EQCM) technique. The change of the complex shear modulus during the film growth and during the doping/dedoping were calculated using the acoustic impedance method. It was found that the films contain only ca. 1 wt% water in the oxidized state which is in accordance with their relatively high storage modulus of 38 MPa and low loss tangent of 0.05. Comparing H3PO4 (pH 1) and NaH2PO4 (pH 8) it was found that the doping/dedoping in the latter is accompanied by a strong increase of the surface roughness which is a direct result of cation exchange present at pH 8. (c) 2006 Elsevier Ltd. All rights reserved.