The surfaces of two tin oxide nanosized powders synthesized by laser evaporation and presenting different average grain sizes were studied by Fourier transform infrared spectrometry. The oxygen adsorption onto activated and oxidized surfaces appeared to be dependent upon the grain size. Moreover, the nature of surface acidic sites probed by pyridine was shown to be dependent on the oxidizing treatment. A simultaneous study of the infrared energy variations and the spectrum perturbations when CO and mixtures of CO and O2 were in contact with the SnO2 samples, showed both the formation of CO2 and the CO reducing effect. The need of an oxygen environment for the surface recovery was clearly brought in evidence as well as the importance of the temperature in the CO sensing mechanism.