The electrodeposition of protective polyaniline (PANi) films on mild steel from aqueous oxalic acid is reported. The electropolymerization of aniline occurs on a surface passivated by the precipitation of an Fe(II)-oxalate layer and leads to strongly adherent films with a controlled thickness and with the same structure as the idealized emeraldine base. When they are dipped in an acidic solution (0.4M NaCl+0.1M HCl) the PANi-coated iron samples exhibit very good protection against corrosion and are much more effective than polypyrrole coated ones.