Perhydropolysilazane (PHPS) films ca. 0.2 μm in thickness were deposited by spin-coating on Si(100) and silica glass substrates. The PHPS films obtained were suspended over aqueous ammonia of various concentrations for various periods of time at room temperature. The Si-H and N-H infrared absorption peaks decreased, while the Si-O-Si peaks increased, and the nitrogen content decreased from 33.6 to 0.5 atomic%, suggesting that PHPS-to-silica conversion occurs on the exposure treatment. The refractive index decreased from 1.56 to 1.48, and the contact angle decreased from 94 to 67° on the exposure treatment. The changes in infrared absorption spectra and in refractive index were more significant when the as-deposited PHPS film was suspended over aqueous ammonia of higher concentrations, indicating that the use of aqueous ammonia of higher concentrations for exposure treatment is more effective in PHPS-to-silica conversion. As the PHPS-to-silica conversion proceeded, the durability of the films in hot water increased, evidenced in both series of films subjected to the exposure for various periods of time and those suspended over aqueous ammonia of various concentrations. When the exposed film was fired at 900°C, the nitrogen content further decreased from 0.5 to 0.01 atomic%, the refractive index decreased from 1.48 to 1.45, and the contact angle decreased from 67 to 29°, all of which approached those of pure and dense silica glass. These changes observed on firing suggest that the silica thin films obtained by the exposure treatment at room temperature are not identical to pure and dense silica glass.