The isothermal change of SnI2 films in air without exposure to light was investigated by weight change and X-ray diffraction analysis for films treated at room temperature or quenched from 50, 100 and 150-degrees-C. At room temperature, alpha- and beta-SnI2 peaks (preferred orientation) similar to those of the as-deposited films were observed after 1-3 h; then, randomly oriented alpha-SnI2 and SnI4 peaks appeared after 4-5 h. Heating accelerated the thermal change. alpha-SnI2 (random orientation) and SnI4 were also detected for films quenched from 50, 100 and 150-degrees-C. The weight loss at 150-degrees-C stopped at 20.4% of the sample weight, which agrees approximately with the value of 20.2% proposed by Sawada and Suzuki (Thermochim. Acta, 232 (1994) 29-36) for the reaction 2SnI2(s) + O2(g) --> SnI4(g) + SnO2(s). Weak diffraction peaks for SnO2 were detected after the SnI4 peaks disappeared.