Pt/SiO2 catalysts prepared by sol-gel processing were studied by transmission electron microscopy, electron diffraction, high-resolution electron microscopy and Fourier-transform infrared spectroscopy. The samples exhibit uniform dispersion of the metal phase with narrow particle size distribution. Treatment with hydrogen in the temperature range 773-923 K yields Pt/Si alloys at lower temperatures and platinum silicides (Pt3Si and Pt12Si5) at higher temperatures. The CO chemisorption shows that with increasing degree of alloying (1) the nu(CO)(l) and the nu(CO)(b) band shifts to lower wavenumbers, (2) the intensity of the nu(CO)(l) band decreases and (3) the nu(CO)(b)/nu(CO)(l) intensity ratio increases. The phenomena are interpreted by simultaneous effects of Pt surface dilution, enhanced electron back-donation from platinum to CO and Pt d-band narrowing.