Thin films of cross-linked poly(4-vinylpyridine) were deposited onto the 4-mercaptopyridine covered gold electrodes of a quartz crystal microbalance. Different amounts of IrCl62-/3- redox anions were then loaded into the films by ion-exchange. The motions of charged and neutral species which occurred during changes in oxidation state were studied with combined mass determinations and cyclic voltammetry or chronocoulometry. During oxidation at low concentrations of redox sites, trifluoroacetate anions were taken up by the film to preserve electroneutrality but neutral molecules, most probably water, were released into the electrolyte. At medium-high concentrations the oxidation process was associated with an ingress of water, while at the highest concentration water influx was smaller and in some cases an outflux was observed. The experimental results are discussed in terms of structural changes and hydration of the films.