The oxygen monolayer coverage obtained by N2O adsorption/decomposition at 363 K on a 0.81% Pt/SiO2 catalyst was compared to those acquired by H-2, O-2, and CO chemisorption. It was equal to the hydrogen monolayer coverage, but about 8% higher than the O coverage acquired by dissociative O-2 chemisorption at either 300 or 363 K, and about 20% greater than irreversible CO coverage. Titration of the O-covered Pt surface obtained via N2O decomposition gave a dispersion value in excellent agreement with that based on H chemisorption, whereas the dispersion value based on titration of an oxygen monolayer obtained by dissociative O-2 chemisorption was slightly lower, as expected. Thus the appropriate stoichiometry for N2O adsorption is Pt-s + N2O(g) --> Pt-s - O + N-2(g). Diffuse reflectance IR Fourier transform (DRIFT) spectra of CO chemisorbed on either clean or O-covered SiO2-supported Pt surfaces gave one predominant peak at 2073 +/- 3 cm(-1) for linearly adsorbed CO and indicated a small extent of bridge-bonded CO. CO adsorption on an O-covered surface also gave a band at 2188 cm(-1), indicative of a small residual amount of oxidized Pt; however, H-2 titration completely removed all surface oxygen. The CO adsorption process is adequately described by Pt-s - O + 2CO(g) Pt-s --> CO + CO2(g). Similar measurements with a commercial 3% Pt/C catalyst revealed that a large, irreversible O-2 uptake occurred on the carbon surface at either 300 or 363 K; however, the amount of titratable oxygen at 300 K was similar to, but slightly higher than, the amount of chemisorbed hydrogen. The oxygen coverage established by N2O decomposition was titrated and found to be slightly lower than the H coverage. DRIFT spectra of CO adsorbed on the Pt/C catalyst gave a single band whose maximum varied from 2019 to 2047 cm(-1), depending on the pretreatment. (C), 2001 Elsevier Science.