Three types of silica-zirconia membranes of different zirconia content were prepared by the sol-gel method to test their stability and H₂ separation performance in hydrothermal conditions. Some silica-zirconia membranes were prepared by “steam-firing” in which firing was carried out in steamed air in order to increase the hydrothermal stability, and then tested in hydrothermal conditions. The activation energy of H₂ and He permeation increased with increasing zirconia content, suggesting that the networks of silica-zirconia matrix were densified with increasing zirconia content. In the hydrothermal conditions, the activation energy of H₂ and He permeation also increased, indicating clearly that silica-zirconia membranes were also densified by water vapor at high temperature. The pores for N₂, Co₂ and CH₄ permeation decreased with the increasing zirconia content and vanished in the hydrothermal conditions, leaving some pinholes. Some silica-zirconia membranes were prepared by firing in steamed air in order to increase the hydrothermal stability. This method appeared to be effective to increase hydrothermal stability of the silica-zirconia membrane.