The surface reactivity of a powder depends on the chemical composition of its surface and on the presence of defects. In the case of nanosized powders, the surface reactivity is enhanced by the increased defect concentration on the surface. On the other hand, the gas sensing properties of a semiconductor material are strongly related to the surface reactivity. In this article, it is shown, by Fourier transform infrared spectrometry, that SnO₂ semiconductor nanoparticles are very sensitive toward CO and that the decrease of the particle size greatly enhances this sensitivity. Comparison is made between particles having an average size of 15 and 8 nm. Surface reactions at the origin of the CO detection mechanism are discussed as a function of the particle size.