The responses of ion-sensitive FETs (ISFETs) with thermally grown SiO//2 gate regions and of electrolyte-SiO//2-Si (EOS) structures to stepwise changes in the pH were studied. A mechanism is also proposed in which one or other hydrogen-bearing species interacts with the surface states at the SiO//2-Si interface. This proposed mechanism is based on the observed time drift in the response of ISFETs and on the changes in the shape of the quasi-static C-V curves of the EOS structures.