Abstract Hydrous oxide films of appreciable thickness, produced on rhodium in base under potential cycling conditions, were observed to be only partially active with regard to charge-storage behaviour. A substantial increase in the charge capacity of such films, apparently to a level encompassing all cations in the film, was observed on prolonged, slow cycling, evidently due to restructuring of the more material in the surface layer. Subjecting the latter to polarization (under either cyclic or steady conditions) at potentials above about 1.6 V resulted in changes in both redox and electrochromic behaviour which were interpreted in terms of partial dehydration of the surface layer. Both the charge-storage and electrical conductivity behaviour of these films are discussed in terms of a recently proposed open polymer (or skeletal) model. The importance of stabilization of the lower oxidation state of the cations, due to complex formation with hydroxide ions, in the oxide growth reaction is also pointed out. Heating the charge-storage layer to about 170°C resulted in a more adherent, probably anhydrous, film which was inactive with regard to charge-storage and electrochromic behaviour.