The reduction of cyclohexane-1,4-dione (1), hexane-1,4-dione (2), and cyclohexanone on a mercury electrode have been investigated. Water and propan-2-ol–water (4 :1) were used as the solvents and tetraethylammonium toluene-p-sulphonate was the supporting electrolyte. Current–potential curves have been measured and preparative electrolyses have been performed. Only compound (1) reacted [up to –2·3 V(SCE)] and yielded the following products: 1,1′-dihydroxybicyclohexyl-4,4′-dione (3), hexane-2,5-dione (2), 4-hydroxycyclohexanone (4), and cis- and trans-1,4-cyclohexanediol [(5) and (6)]. At all potentials the yield of cis-compound (5) exceeded that of trans-compound (6). The ratio cis(5):trans(6) decreased with increasing cathodic potential. Possible mechanistic paths leading to the different products and adsorption phenomena involved in their formation are discussed.