The reduction of hexamminecobalt(III) at the mercury-water interface is strongly affected by the presence of a condensed tetrabutylammonium film. When the anions present are fluoride and/or tetrafluoroborate, electron transfer through the film proceeds via an inner-sphere mechanism, i.e. the film must be opened so that hexamminecobalt(III) can make direct contact with the metal. In the presence of bromide anions, an outer-sphere pathway is indicated, probably involving bridging by bromide ions. Unusual current-time transients of the reduction of hexamminecobalt(III) in the presence of tetrabutylammonium ions are correlated with the kinetics of film formation.