The operation principle of a semiconductor nanowire (NW) ion-sensitive field-effect transistor (ISFET), denoted for pH sensing, is studied within the framework of this work. The physical processes in the system are mathematically modelled and presented in details. The dependences of the NW ISFET current-pH characteristics on NW geometrical and physical parameters are analyzed. The plots of the ISFET sensitivity versus pH at different NW radii, the thicknesses of the oxide layer, and the NW doping densities are presented. The obtained results are in qualitative agreement with the experimental data.