Electron transfer reduction of p-benzoquinones by cobalt tetraphenylporphyrin is enhanced significantly by the presence of o-bis(phenylcarbamoylmethyl)benzene (o-L) due to the regioselective hydrogen bond formation between the corresponding semiquinone radical anions and o-L, whereas m- and p-isomers (m-L and p-L) have no effect on the electron transfer equilibrium or the rate.