Electrochemical oxidation of two isomeric coumarins, umbelliferone (UF, 7-hydroxycoumarin) and benzotertonic acid (BA, 4-hydroxycoumarin), were comparatively studied in aqueous buffer solutions by cyclic voltammetry, in situ long-path-length thin-layer UV–vis spectroelectrochemistry and ex situ ATR-FTIR spectrometry. Both the coumarins undergo the completely irreversible oxidation but following totally different oxidation mechanisms. The 7-OH but not the 4-OH group can contribute to antioxidative activity of coumarin via an electron transfer mechanism. Electro-oxidation of UF occurs at the 7-OH position and produces an insulating polymer film at the electrode surface, which probably consists of a poly(ethylene oxide) backbone with coumarin side groups. The toxicity-related coumarin 3,4-epoxide is a possible intermediate in the UF oxidation. Electro-oxidation of BA occurs at the C3 C4 double bond, also yielding a non-conductive film at the electrode surface. In this process salicylaldehyde as the possible intermediate undergoes further oxidation to form the poly(aryl ether) film. The knowledge of the mechanisms of UF and BA oxidation should be helpful in understanding the roles and conversion of coumarins in their biological and chemical processes.