Electropolymerisation of pyrrole on mild steel was carried out in 0.01 M sodium molybdate and 0.1 M pyrrole solution at constant current 1.5 mA cm(-2). The film is homogenous and adherent. The corrosion protection of polypyrrole films containing inhibitor anion molybdate as dopant anion was investigated with open circuit potential (OCP) in 0.1 M NaCl. The passivating effect of molybdate was demonstrated. The mobility of anion was investigated by cyclic voltammetry (CV), electrochemical quartz microbalance (EQCM) and electrochemical impedance spectroscopy (EIS) measurements.