The electrical impedance of quartz crystals was measured during electropolymerization of poly (2,2'-bithiophene) (PBT) and poly-(3,4-ethylenedioxythiophene) (PEDT) from acetonitrile and propylene carbonate. Due to the viscoelastic behavior and the high surface roughness, application of the Sauerbrey equation to relate frequency shifts to mass changes leads to systematic errors. A novel procedure is presented to calculate the mass changes, the viscoelastic properties (complex shear modulus), and the surface roughness of the conducting polymer layers from the shift of the resonance curve of the quartz crystal. The dependence of the surface morphology of PEDT and PBT on the preparation conditions could be monitored in situ. The values of the complex shear moduli show that these polymers have rubber-like behavior. The damping increase of the quartz at the initial stages of the polymer deposition is mainly a surface roughness effect. As the polymer thickness increases, internal friction (viscoelasticity) also begins to contribute to the damping.