We investigated the formation of Ag nanoparticles at the interface of an Ag thin film with a polyethylene terephthalate (PET) substrate under intense visible light irradiation at elevated temperatures. Using dc magnetron sputtering, Ag thin films were prepared on a, PET substrate at room temperature. The interface was exposed to visible light from a Xe lamp with an intensity of 500 mW/cm(2) through the PET substrate. Cross-sectional views of the interface were observed with the use of a transmission electron microscope before and after the irradiation. In order to avoid the deterioration of the PET substrate itself, the component of the ultraviolet (UV) light was excluded by placing a UV-cut filter between the light source and the specimen. The total reflectance was measured with a spectrophotometer equipped with an integrating sphere. The reflectance of the Ag film through the PET was found to decrease due to the formation of Ag nanoparticles at the interface after the visible light irradiation. Deposition of a few monolayers of metals such as Ti and W on PET before the deposition of Ag was quite effective in retarding the formation of the Ag nanoparticles and in maintaining the high reflectance of the Ag thin film. With the increase in the intensity of the light and the rise of the temperature, the decrease in the reflectance was accelerated. By fitting the curves of the decrease in the reflectance at different temperatures to a stretched exponential function, an apparent activation energy of the reaction corresponding to the decrease. of the reflectance was estimated as 50 kJ/mol. We will also report the light intensity dependence of the decrease in reflectance.