The reaction and poisoning mechanism of SO2 with La0.9Sr0.1CoO3 model catalysts have been investigated. The structure and the chemical states of the model catalysts have been studied by using AES, XPS and XRD techniques. The results indicated that SO2 diffused into the La0.9Sr0.1CoO3 film during poisoning. La-2(SO4)(3) species was formed on the surface of the film and La-2(SO4)(3), of SO2. The concentration of sulfur in the film layer was related to the reaction temperature and time. After the sample was poisoned for a fairly long time, the distribution of sulfur in the La0.9Sr0.1CoO3 layer became homogeneous, suggesting that a dynamic equilibrium was achieved between the poisoning reaction and the decomposition of the sulfates. XRD and catalytic activity test results proved that the destruction of perovskite structure and the formation of sulfates were the main causes of deactivation.