Spin-on perhydropolysilazane (PHPS) thin films were converted into mechanically hard silica thin films by an exposure to the vapor from aqueous ammonia. Infrared absorption and X-ray photoelectron spectroscopic analyses were conducted to clarify the details of the PHPS-to-silica conversion and the nature of the silica thin film products. The PHPS-to-silica conversion was found to proceed rapidly between 2 and 3 h of exposure via a reaction-limited process, where the refractive index and the pencil hardness greatly decreased and increased, respectively. Finally, the O/Si mole ratio close to 2 was achieved, which has never been realized in literature for PHPS-derived silica thin films. It was also found that the condensation of Si–OH groups proceeds in films immediately after PHPS hydrolysis, which is similar to the base-catalyzed hydrolysis–condensation reaction of silicon alkoxides. Although the silica thin films obtained had refractive indices similar to that of silica glass, high pencil hardness over 9H on Si(100) substrates, and O/Si mole ratios close to 2, it was concluded that they are nonidentical to silica glass, containing trace amounts of Si–OH and Si–H groups.