Electrochemical carboxylation of vinyl triflates using a magnesium anode, as a reactive metal electrode, was investigated. Alkyl-substituted vinyl triflates were electrolyzed in the presence of an atmospheric pressure of carbon dioxide to give beta-keto carboxylic acids as a sole products. On the other hand, electrochemical carboxylation of phenyl-substituted vinyl triflates gave alpha,beta-unsaturated carboxylic acids exclusively. These divergent electrochemical carboxylation resulted from chemoselective cleavage of O-S or C-O bond of vinyl triflate, which was affected by the presence or absence of phenyl substituents in the structure of starting vinyl triflates.