The time dependence of interfacial potentials is analysed approximately in terms of the space charge polarization and the diffusion within the membrane for ion-conducting thin films (less than or equal to 100 nm) used in field effect-based sensors. Results are experimentally verified at aluminosilicate thin films fabricated by ion implantation. The polarization leads to an initial transient and a potential displacement by about 52-59 mV per 1 V polarizing voltage. Bulk membrane diffusion is characterized by a diffusion coefficient for sodium ions of 5 x 10(13)-7 X 10(-13) cm(2) s(-1).