Electrochemical behaviour of triactivated thiopyrans has been investigated in protic medium. Electroreduction of 4-dimethylamino-2,3,5-trimethoxycarbonyl I first led to 2,3,5-trimethoxycarbonyl-2H-thiopyran IV, with elimination of dimethylamine; further reduction at more negative potential gives equimolar mixture of 2,3,6-trimethoxycarbonyl-3,6-dihydro-2H-thiopyran V and 2,3,6-trimethoxycarbonyl-5,6-dihydro-2H-thiopyran VI. The same mixture is obtained by reduction of either 4-dimethylamino-3,5,6-trimethoxycarbonyl-3,4-dihydro-2H-thiopyran II or 3,5,6-trimethoxycarbonyl-2H-thiopyran III. Intermediate formation of a delocalised carbanion is postulated in order to explain these results.