Thin, nonporous films of bismuth ruthenium oxide having the pyrochlore crystal structure were prepared by ion beam deposition. The films were grown on metal disks machined to fit into a rotating : ring-disk electrode. The electrocatalytic properties of these films for O-2 reduction and evolution were studied in 1 M KOH using the rotating ring-disk electrode method in conjunction with spectroscopic studies of the surfaces before and after electrochemical analysis. The crystalline pyrochlore film was found to be an extremely poor oxygen reduction catalyst, but it could be activitated in situ by a procedure that created a highly porous amorphous structure by dissolution of bismuth cations into the solution. We found in separate experiments that bismuth ions in solution increased the oxygen reduction activity of a nonporous ruthenium oxide surface, and suggest that there is an analogous effect on the activity of the residual, ruthenium-rich oxide surface by bismuth ions in solution following activation.