In order to obtain a fundamental understanding of the corrosion behavior of tin in atmosphere, in situ IR-RAS (infrared re-flection absorption spectroscopy) measurements were performed on tin in humid air containing SO2 and NO2 at room temperature.Time-resolved in situ IR spectra in air of 90% RH (relative humidity) containing 1×10-5-2.2×10-5 SO2 suggested that the tin oxidelayers worked as a protective film and no significant corrosion occurred. The conosion products in air of 80%-90% RH containing1×10-5-2.2×10-5 NO2 were SnO2, SnO, nitrate and hyponitrite. The synergistic effect of SO2 and NO2 on corrosion of tin was not ob-served in air of 90% RH containing 0.84×10-6 SO2 and 1.8×10-6 NO2.