Two copper (II) complexes L-1 Cu and (LCu)-Cu-2 (H2L1= 4,9-dimethy1-2,11-dioxy-5,8-diazacycldode-2,4,8,10-tetraene; H2L2 = di ethy1-3,8-dimethy1-4,7-diazadeca-2,8-diene-1,10-dioat) were prepared and displayed good catalytic efficiency for the oxidations of Mandelic acid (MA), 4-hydroxymandelic acid (4-HMA) and 4-methoxymandelic acid (4-MMA) by H2O2 in aqueous solution at 35 C. The apparent first-order rate constants k(obs) of oxidations of MA, 4-HMA and 4-MMA were obtained in the range of pH 2.5-4.5. It was great interesting that the complexes LCu displayed excellent catalytic ability for the selective oxidation of Mandelic acids into aromatic aldehydes (S > 98%). The active species generated by reacting copper (II) complexes LCu with H2O2 was characterized by UV vis, electrospray ionization mass spectrometry (ESI-MS) and cyclic voltammetry (CV). The LCu1-(OOH)-O-center dot species was suggested as the possible active species. The geometric configurations of LCu and the LCu1-(OOH)-O-center dot species were calculated with Density Functional Theory (DFT) method. The LCu1-(OOH)-O-center dot species is stable and possesses relatively high oxidation potential in aqueous solution at ambient temperature. This study also provides a new insight for the degradation of lignin. (C) 2013 Elsevier B.V. All rights reserved.