The models proposed in the literature on the mechanism of operation of inorganic-gate pH-sensitive ISFETs can be divided in three categories: those involving changes at the Si/insulator interface, those involving bulk ionic diffusion and those based on reactions of surface sites. The first two categories imply a time response limited by diffusion through the gate insulator. Time response data on Al2O3-gate ISFETs show that the inrtinsic response time is of the order of a few milliseconds or faster. Published data for other insulators are similar. The diffusion coefficient for H+ diffusion in SiO2 is much too low to explain this fast response, and for Al2O3 and Si3N4 no H+ movement can be detected at low temperatures. Gel layer formation cannot increase ionic mobility sufficiently to explain the observed response times. Therefore we conclude that surface effects must be responsible for the fast pH response. We propose that an additional slow response resulting in hysteresis as observed in SiO2-gate ISFETs, as well as a decreased sensitivity for higher pH values, are due to the presence of OH sites buried beneath the surface. These interior OH sites can be created by steam oxidation or by exposure to the aqueous electrolyte.