Metal co-catalysts are essential for enhancing photocatalytic performance, especially in reduction reactions using semiconductor photocatalyst materials as a consequence of the reduced recombination kinetics of charge carriers by spatial charge separation. Generally Au, Pd, Pt, and their alloys are more promising candidates than Ag for photocatalytic H-2 evolution experiments, although Ag can trap more electrons having more negative reduction potential than that of Au, Pd, and Pt. Here we have synthesized and examined well-defined Au, Ag, and core-shell structured Au-Ag nanoparticles as co-catalysts for TiO2 in photocatalytic H-2 evolution. By varying the dissolved oxygen in the reaction suspension, we found that selective photocatalytic reduction can be achieved by fine tuning the co-catalyst materials. Whilst Au NPs are superior for proton reduction, Ag NPs exhibits excellent performance for oxygen reduction. All core-shell structured Au-Ag NPs show non-selectivity in photocatalytic reduction of proton and oxygen. (C) 2015 Elsevier B.V. All rights reserved.