The effects of redox state (Os[II] vs. Os[III]) and counter ion (perchlorate vs, p-toluenesulphonate) on the polymer and solvent density distributions in films of electroactive [Os(bipy)(2)(PVP)(10)Cl](+) were determined using neutron reflectivity. The changes in polymer structure upon immersion in solvent and upon redox cycling the film in these electrolytes were also investigated. The as-prepared dry polymer film swelled appreciably upon immersion in aqueous solutions. Using p-toluenesulphonate as the counter ion caused pronounced changes to film structure upon both redox cycling and holding it in a fixed oxidation state; the perchlorate anion was found to be much less disruptive. Surprisingly, the effect on film structure of changing anion was much more pronounced than the effect of changing film redox state, i.e. charge density.