The adsorption, decomposition and oxidation of CH3Cl have been investigated on pure and chromia-doped SnO2 catalysts by means of infrared spectroscopy, mass spectrometric and gas chromatographic analysis. Catalyst samples have been characterized by X-ray photoelectron spectroscopy. It was found that the Cr3+ ions incorporated in the surface layer of SnO2 are oxidized to higher valent chromium ions. In harmony with the determination of active oxygen content, samples calcined at 873-973 K contained the largest concentration of higher valent chromium ions. Infrared spectroscopy showed that CH3Cl interacts with the OH groups of SnO2 at 200-300 K to form H-bridge-bonded alkyl chloride. The decomposition of CH3Cl occurred slowly on pure SnO2 above 573 K to give CH4, H2O, CO and HCl. The oxidation of CH3Cl on SnO2 proceeded at a measurable rate at 400-440 K. Incorporation of a small amount of chromium ions into the SnO2 increased the rate of both the decomposition and the oxidation by a factor of 10-16. The enhanced activity of chromia-doped SnO2 is attributed to the high reactivity of chromium ions in the surface layer of SnO2.