Polyethylenedioxythiophene films were electropolymerized potentiostatically (E = 1.1-1.3 V/SHE) at a gold electrode covering an EQCM from solutions containing monomer, lithium perchlorate and non-ionic surfactant (polyoxyethylene-10-laurylether). The ion exchange was studied by EQCM during redox cycling at a scan rate of 20-50 mV s(-1) in 0.5 M LiClO4 + 0.5 M LiCl + 0.5 M Na toluenesulfonate solutions in the potential range - 0.5 < E < 0.8 V (SHE). Mass versus charge curves display a hysteresis. During the reduction scan the ion content of the film is in equilibrium and is defined by the potential, whereas during oxidation the mass lags behind the charge due to slow water exchange. The reduction scan can be described by equilibrium theory supposing that about 40% of counter-ions remain bound in the film. During polymerization at E greater than or equal to 1.2 V, the mass gain calculated from the frequency change slows down more rapidly than is estimated from the current. This may be attributed to the viscosity of the growing film with acoustic decay length of 2.45 mum.