The polyanion of polystyrene sulphonate (PSS) combined in polypyrrole (PPY) as the counterion during the electrochemical polymerization of pyrrole is shown to be trapped permanently in the PPY network during subsequent redox switching of the PPY–PSS in a supporting electrolyte, and thus cation (co-ion) exchange is a primary feature of the process. In association with small cations such as K+ and favourable anions such as NO–3, electrochemical charge transport within the PPY–PSS films is reversible and fast with Dct= 6 × 10–8 cm2 s–1. A previously unreported effect of electrolyte anions, however, has also been observed. The effect is profound for the unfavourable SO2–4 anion and is reflected in a significant change in the electrochemical and EPR behaviour of the conducting polymer compared with that of the favourable anions. EPR results indicate that the oxidation level of PPY during film deposition becomes lower as the film increases in thickness. During redox switching in a supporting electrolyte, disproportionation of PPY radical cations is significant at high oxidation levels. PPY–PPS is shown to be very stable electrochemically and capable of scavenging transition-metal ions, which can be used in stripping analysis of these metals.