The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MQ(x)/Al2O3 and MOx/TiO2 supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150-500 degrees C, using a feed composition consisting of 3% CO an 10% H2O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 degrees C is 1-2 orders of magnitude higher when supported on "reducible" compared to "irreducible" metal oxides. For composite Pt/MQ(x)/Al2O3 and Pt/MOx/TiO2 catalysts, it is shown that the presence of MO, results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al2O3 or PUTiO2, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MO,. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H2O, 20% H-2 and 6% CO2, showed that certain composite Pt/MOx/Al2O3 and Pt/MOx/TiO2 promising candidates for the development of active WGS catalysts suitable for fuel cell applications. (C) 2007 Elsevier B.V. All rights reserved.