Proton conduction in sol-gel-derived porous glasses was investigated and is related to the pore structure. The porous P 2 O 5 -SiO 2 glasses absorb water under ambient conditions and consequently increase the electrical conductivity. In glasses having pores ∼5 nm or smaller, the water molecules diffuse through the pores and totally fill the pores. The motion of the water molecules is restricted in small pores, and the absorbed water molecules are well retained even at low humidity. The conductivity of this glass was high in the temperature range down to -20 °C. On the other hand, in glasses with large pores, no confinement effect occurred for the water molecules and the conductivity suddenly decreased below 0 °C.