The ability of manganese(II) complexes with substituted terpyridine (terpy) ligands to activate hydrogen peroxide and to catalyze oxidation of the substrates Morin and Trolox C was studied in aqueous alkaline solution. Introduction of pi-donor substituents as hydroxy or amine groups in the 4-positions of the pyridine rings resulted in an enhanced catalytic activity compared to the unsubstituted terpyridine-manganese(II) complex. However, the catalytic activity depends critically on the substitution pattern. Differences in the activity of catalysts towards Morin and Trolox C are discussed with respect to the mechanism of oxidation. Dissolution of isolated complexes with a ligand-to-manganese ratio of 1: 1 in aqueous buffer results in formation of a mixture of 1:1 and 2:1 species. The 1:1 species was found to be the catalytically more active precursor, suggesting that free coordination sites are important for a high catalytic activity. Activity was found to be pH dependent and was highest at pH similar to10. All complexes also catalyze the disproportionation of hydrogen peroxide into oxygen and water. (C) 2003 Elsevier Science B.V. All rights reserved.