Samples of platinum crystallites finely dispersed on silica and silica-alumina were prepared by impregnating a series of SiO2-Al2O3 supports with aqueous PtCl4 solutions. Reduced platinum samples were oxidized by dioxygen to examine variations of platinum-oxides (Pt-O-x) species formed with the oxidation temperature (T-O). Both PtO and PtO2 species were verified to coexist on oxidized Pt/SiO2 samples according to characterizations by techniques of X-ray photoelectron spectroscopy and temperature-programmed reduction (TPR). TPR spectra of oxidized platinum species dispersed on SiO2-Al2O3 supports exhibited six peaks of different reduction temperatures (T-r). These peaks were assigned to PtO and PtO2 species dispersed in silica-rich grains (T-r = -40 and 20 degrees C, respectively), in alumina-rich grains (T-r = 50 and 90 degrees C), and in pores of the grain boundary (T-r = 110 and 130 degrees C), respectively, The contribution of these six species varied with both the temperature of oxidation treatments and the Al2O3/SiO2 ratio of sample supports.