Poly-3,4-ethylenedioxythiophene composite films with tungsten oxide (PEDOT/WO₃) were obtained by potentiodynamic deposition of tungsten oxide from acidic metastable solution of isopolytungstates into poly-3,4-ethylenedioxythiophene film, pre-deposited on an Au-quartz crystal. The electrochemical deposition of tungsten oxide into poly-3,4-ethylenedioxythiophene was investigated by electrochemical quartz microgravimetry (EQCM), the masses of PEDOT and tungsten oxide deposits were estimated. The electrochemical behavior of PEDOT/WO₃ composite films with different masses of deposited tungsten oxide was studied by cyclic voltammetry and EQCM in the 0.5 M sulfuric acid electrolyte. The change of the fraction of tungsten oxide in the PEDOT/WO₃ composite film results in the change of the slope of the Δf–E dependency in the potential region, corresponding to W^VI/W^V redox process. It is due to an oppositely directed mass transport (anion and cation) during the redox process in the composite film. The average molar mass of transferred species involved in the redox process in tungsten oxide was estimated to be 26 ± 4 g mol⁻¹. It was shown that the fraction of electrochemically active tungsten oxide deposit is dependent on its mass in the composite, with increase of the mass of tungsten oxide in PEDOT/WO₃, the fraction of electroactive tungsten oxide WO₃ decreases due to decrease in its active area.