Gasochromic films can reversibly switch their optical transmittance upon alternating exposure to diluted H-2 and O-2 gases. These films are usually prepared by physical methods such as sputtering and pulsed-laser deposition. Here, we report a new method using UV irradiation for low-temperature chemical fabrication of Pt-WO3 gasochromic switchable films. In this method, an amorphous film of WO3 is produced, and the photocatalytic properties of the WO3 are utilized for the reduction of the Pt2+ precursor to Pt particles. This method also generates a large number of voids in the film at 100 degrees C. The initial transmittance of the Pt-WO3 films was extremely high (approximately 80%) over the visible to the near infrared region, and the optical change in the transmittance upon exposure to H-2 gas or air was large (approximately 70% at a wavelength of 670 nm). In addition, the switching time of the films was as fast as 5 s, and the switching durability of the films was preserved for over 1500 cycles.