The electrolysis of alicyclic ketones (5, 6, 7, and 8-membered rings) with a carboxylic ester and a bromomethyl group was carried out in N,N-dimethylformamide in the presence of a catalytic amount of heptamethyl cobyrinate perchlorate under various reaction conditions. The ring-expansion reaction largely proceeded at -2.0 V vs sce for all the substrates. The electrolysis plausibly proceeds as follows : the Co(II) complex as a catalyst is electrochemically reduced to the Co(I) species; the corresponding alkylated complex is generated by reaction of the supernucleophilic Co(I) species with an alicyclic bromide; the alkylated complex is subsequently decomposed by electrolysis to afford the final products; and the cobalt complex acts as a mediator repeatedly.