Polypyrrole coatings have been successfully electrodeposited on steel substrates from aqueous oxalate solutions. In acidic solutions, the electrodeposition process was characterized by three distinct stages. In this work the electrodeposition mechanism in acidic solutions was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and reflection-absorption infrared spectroscopy (RAIR). Our results reveal that deposition of a thin layer of FeC2O4. 2H(2)O crystal layer first established the passivation of the steel substrate at the end of the second stage, but the FeC2O4. 2H(2)O formed was subsequently decomposed in the initial part of the third stage and electropolymerization of pyrrole occurred directly on the steel surface. The proposed electrodeposition mechanism is also consistent with the results of lap shear tests.