Electrochemical activity of a cast film of a water-soluble polythiophene, poly(3-(3’-thienyl)propanesulphonic acid) (PTS), is examined in various electrolyte solutions. The PTS film is electroactive in 0.1 M tetra(n-butyl)ammonium perchlorate (TBAClO(4))+ acetonitrile. In contrast, the film is inactive in 0.1 M LiClO4 + acetonitrile, even in the presence of TBA(+), while addition of water (above 0.1 M) activates it. These characteristics are related to the film swelling; the PTS film is electroactive only when it is swollen. Water swells the film rapidly and TBA(+) swells the film much more gradually. In the 0.1 M TBAClO(4) + acetonitrile, potential cycling accelerates the film swelling owing to break-in behaviour. Well-swollen films do not exhibit break-in.