In order to investigate the effects of manipulating chemical structures in naphthalene sulphonate-based polymermodified electrodes, the amino and methyl groups were incorporated into a polypyrrole matrix. N-Methylpyrrole was polymerized anodically on Pt electrodes from acetonitrile-water solvents with various volume ratios together with napthalene sulphonate compounds as dopants in the polymerized film. Aminonapththalenesulphonate compounds were also used as dopants in the polypyrrole film. The e.m.f. response to cationic species was tested for solutions of alkali and alkaline earth metal chlorides. The response decreased on introducing electron-donating groups into the structure, either methyl groups in the polypyrrole network or amino groups in the naphthalene moieties. The Nernstian response of the electrodes appeared in the limited range of mixing ratios of acetonitrile-water solvents used for elecrochemical polymerization. The best electrodes revealed single/double charged cation selectivities of log K K,Mg pot = −3.4. It was confirmed by X-ray photoelectron spectroscopic analysis that the amount of the naphthalenesulphonates with respect to N-methylpyrrole was affected strongly by the acetonitrile-water composition of the solvents. A sulphur / pyrrole nitrogen ratio of >0.20 was required for the appearance of the e.m.f. response to cationic activity. Amino groups in the film appear to have counteracted the role of sulphonate groups as cationic sites.