The pH sensitivity of ISFETs arises from interactions of protons with ISFET gate surface sites. This sensitivity is described by a new simpler model with the intrinsic buffer capacity and the differential capacitance as key parameters. The obtained expression is independent of the models used for the chemical surface equilibria and the charge profile in the solution. The general expression for the sensitivity is elaborated using the site-binding theory and the Gouy-Chapman-Stern theory. The relatively high sensitivity of Ta2O5 ISFETs is explained using this elaborated theory. It is shown that the electrolyte concentration has almost no influence on the sensitivity of Ta2O5 ISFETs.