Electrodeposited polypyrrole films prepared with paratoluenesulfonate (pTS), dodecylsulfate (DDS) and perchlorate anions were treated with acidic and basic solutions, and their structure was investigated by C-13 solid state n.m.r. spectroscopy. This technique has confirmed that pTS is completely removed from the film in both acidic and basic solutions whilst DDS is only partially removed and tends to decompose upon treatment with H2SO4. The appearance of shoulders at 143 ppm upon treatment with 0.5 M base indicates formation of a quinoid pyrrole structure. Substitution on the beta-carbon by OH cannot be confirmed from the present spectra. Stronger base causes a more dramatic change in the polypyrrole backbone with an obvious increase in the electron density on the beta-carbons, consistent with the reduction of the carbon backbone. There is no indication of quinoid units in this case. Acid treatments result in considerable broadening of the main 127 ppm polypyrrole peak.