The electrochemical behaviors of a tris(2-pyridylmethyl)amine copper complex (Cu-tmpa) in aqueous buffer solutions were examined. Acid-base and redox equilibria were clarified using cyclic voltammetry and ESR spectra. The pK(a) values of both Cu-II-tmpa and Cu-I-tmpa were determined based on the pH-E degrees' diagram of Cu-tmpa. The slopes in the diagram strongly suggest the presence of hydroxo-bridged dimer complexes of Cu-II-tmpa. This notion is supported by ESR spectra. The oxygen reduction reaction (ORR) by Cu-tmpa was also examined over a wide range of pHs. The ORR current of Cu-tmpa decreased significantly above pH 12.0. This pH value coincides with the pK(a) value of Cu-I-tmpa. The decrease in ORR current in basic solution is attributed to the formation of a hydroxo complex of Cu-I. These findings are important for the application of Cu complexes as cathodes in fuel cells. (C) 2016 Elsevier Ltd. All rights reserved.