An oligomer solution was prepared from methyltriethoxysilane. CH3SiO3/2 gel film was prepared by pouring the oligomer solution onto an aqueous solution. To improve this preparation method, the supporting solution was substituted by aqueous ammonia solutions after gelation. The structure of these films was investigated by FTIR attenuated total reflectance (ATR). This treatment shortened the time for the oligomer solution to become rigid on the supporting solution and caused a decrease of the number of Si-OH bonds in the gel film. The results are discussed based on the mechanism of condensation reaction in the film.