A transparent magnesium tin oxide thin film was prepared on a 330-nm-thick indium tin oxide (ITO)-coated glass substrate. UV irradiation led to darkening of the film. The colored film could be reversibly decolorized by an appropriate thermal treatment. As this coloration-decoloration phenomenon did not occur without the ITO layer and the magnesium tin oxide layer was too thin to absorb the incident light sufficiently, the coloring process is thought to occur in the ITO layer. Ab initio calculations indicated that the darkening of the ITO layer was the result of oxidation caused by the magnesium tin oxide layer.