Polypyrrole films doped with molybdenum trisulfide and tetrathiomolybdate anions have been prepared from an aqueous solution containing pyrrole and ammonium tetrathiomolybdate in the presence of tiron, (4,5-dihydroxy-1,3-benzenedisulfonic acid), in the deposition solution. In such a solution the electrodeposition of polypyrrole doped with tetrathiomolybdate anions is in competition with the electrodeposition of molybdenum trisulfide by oxidation of tetrathiomolybdate anions. Our results indicate that the relative amounts of polypyrrole and molybdenum species can be varied over larger ratios when tiron is present in the deposition solution. The variation of the amount of molybdenum species can be explained by considering the number of electrons required to generate one mole of tetrathiomolybdate as dopant for polypyrrole and one mole of molybdenum trisulfide; 14 electrons are required per mole of tetrathiomolybdate incorporated in the polymer in comparison to 2 electrons per mole of molybdenum trisulfide generated directly by electrochemical oxidation of tetrathiomolybdate. Tiron acts as a catalyst for the deposition of polypyrrole and in this case the relative amount of polypyrrole in the composite film electrode is larger. For higher tetrathiomolybdate concentration, the direct formation of molybdenum trisulfide occurs preferentially and the catalytic effect of tiron is less important.