In-situ electron spin resonance studies of poly(orthotoluidine) and poly(2-ethylaniline) were reported in aqueous acid solutions of various acid concentrations. The results indicate the formation of radical cations between the first and second redox waves in the cyclic voltammogram. The range of acid concentrations where the radical cation is stable decreases in the following order: polyaniline > poly(orthotoluidine) > poly(2-ethylaniline).
