This paper presents a COMSOL Multiphysics 2D axisymmetric model of an pH-ElecFET (pH-sensitive electrochemical field effect transistor) microsensor. This device combines an integrated microelectrode with a pH-sensitive chemical field effect transistor (pH-ChemFET). Thus, by triggering electrolysis phenomena owing to the integrated microelectrode, associated local pH variations in microvolumes are monitored thanks to the pH-ChemFET microdevice. Taking into account (electro) chemical reactions and diffusion phenomena in liquid phase, the proposed model points out the role of the ElecFET geometrical design (microelectrode width w, gate sensitive radius re and distance between the pH-ChemFET gate and the microelectrode d), as well as polarization parameters, (polarization voltage Vp and time tp), on the microsensor response. It is first applied to water electrolysis in order to validate pH impulsional variations in microvolume. Then, oxidation of hydrogen peroxide in phosphate buffer (PBS, pH0=7.2) solutions is studied, evidencing the H2O2 potentiometric detection in the [10–100mM] concentration range. This developed model paves new ways for sensor applications, opening several new opportunities for pH-ElecFET devices for H2O2-related enzymatic detection of biomolecules.