A conductive polypyrrole (PPy) film was successfully synthesized in a homogeneous supercritical carbon dioxide (scCO(2))/acetonitrile (AN) system. The occurrence of a homogeneous supercritical state was confirmed by observations of the phase behavior of the system through a high-pressure cell with a viewing window. The concentration of a supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF(6)), significantly changed the phase behavior of the scCO(2)/AN system. The polymerization rate of the film in that system decreased with further addition of CO2. This result suggested that the low Viscosity of scCO(2) did not play an important role in improving the growth rate of the PPy film. The low polymerization rate might have been due to the electron-transfer resistance arising from the low dielectric constant of scCO(2)/ AN mixture. The roughness of the film prepared in the homogeneous scCO(2)/AN system was 1/10 that synthesized in AN itself as a solvent. The slow growth of film and the high diffusion rate of the monomer seemed to account for the smooth flat film formation.