The response characteristics of palladium‐gate metal‐insulator‐silicon field‐effect transistor (MISFET) structures to hydrogen are shown to be logarithmically related to the partial pressure of the gas. It is proposed that these palladium‐gate MIS structures should be considered as electrochemical devices where the metal/insulator structure behaves as a gas electrode. The results presented were obtained for hydrogen in air and therefore relate directly to the use of these devices as sensors for use in the ambient environment.