We have characterized electrically conductive polymer, polypyrrole (Ppy), Ðlms on mild steel with atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). The Ppy Ðlms were polymerized electrochemically from sodium p-toluene sulphonate (ToS) water-ethanol solution. A very good corrosion protective e †ect of those Ðlms on mild steel has been observed previously. After drying in air, thin Ppy Ðlms (¿200 nm) were highly adhesive, whereas thicker Ðlms (¿7.5 lm) could easily be peeled o † from the metal. All Ðlms had a nodular surface structure. Thin Ðlms were very smooth and their roughness increased with their thickness. The AFM study of the interface between the metal and the polymer showed areas of di †erent morphology, which suggests that adhesive and cohesive failure of the polymer Ðlm occurs. A strong variation of the chemical composition at the interfacial layer was observed. A sulphur-rich layer was formed at the interface, indicating that the counter-anions (ToS) were accumulated on the surface of the metal. High-binding energy iron and imine-type nitrogen were detected in a thin Ppy Ðlm. Moreover, thin Ðlms were substantially overoxidized. The number of anions incorporated into the Ppy matrix varied as a function of Ðlm thickness : the "doping levelÏ was about 30% for thicker Ðlms and only 12% for thin ones. Nitrate counter-anion-containing Ppy Ðlms were also studied in some cases for comparison.