The new copper(II) thiosemicarbazone complex CuL (H₂L = 4-{bis[4-(p-methoxyphenyl)-thiosemicarbazone]}-2,3-butane) was synthesized and structurally characterized. Its electrochemical behavior in N,N-dimethylformamide (DMF) was examined. This complex proved active for homogeneous oxygen reduction reaction (ORR) in DMF in the presence of organic acids. The activity and selectivity for ORR were found to be dependent on the nature of the proton source (phenol or acetic acid). This study confirms the previously reported free energy linear correlation between selectivity and overpotential requirement [Passard et al., J. Am. Chem. Soc. 2016, 138, 2925–2928] and suggests that additional descriptors are required for a full understanding of the catalytic ORR behaviour.