The reaction and poisoning mechanism of SO2 with LaCoO3/Al2O3 film model catalyst have been investigated using AES, XPS and XRD techniques. Some sulfates such as La-2(SO4)(3) were formed on the surface of the poisoned sample. SO2 diffused into LaCoO3 film and reacted with LaCoO3 to form La-2(SO4)(3), La-2(SO3)(3), La2O2SO4 and CoO species in the interior of the LaCoO3 layer. The perovskite structure of LaCoO3 was destroyed by invading of such sulfur and some new phases were formed, which led to the loss of the catalytic activity. The concentration of sulfur in the catalyst layer increased with the temperature and the time. After the sample was poisoned at a high temperature above 700 degreesC or for a period of time longer than 9 h, the distribution of sulfur in the LaCoO3 layer became homogeneous and the concentration was fairly low, suggesting that an equilibrium was achieved between the poisoning reaction and the decomposition of the sulfurous salts.