The optical sensitive characteristic of SnO 2 and VO x thin films to the gas SO 2 is investigated with photometric and ellipsometric methods. It is found that thin films of the Pd doped SnO 2 and no dopant VO x have good optical sensitive characteristics to the gas SO 2 in the visible range, and the Zr doped SnO 2 film in the near infrared area. The results show that the change of the optical characteristic is directly related to the variation of optical gap and the concentration of free carrier in the thin films.