In this study, we developed a super Nernstian pH sensitivity of InSnxOy (ITO) sensing film deposited on PET flexible through a simple sol-gel method for an extended-gate field-effect transistor (EGFET) pH sensor. We employed X-ray diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to characterize the structural, morphological and chemical features, respectively, of the films prepared under three Sn concentrations (10, 15 and 20 mol%). The ITO-based EGFET device processed at the 15 mol% Sn concentration exhibited the highest sensitivity of 62.04 mV/pH, the lowest hysteresis voltage 2.60 mV and the smallest drift 1.08 mV/h among these concentrations. We attribute this behavior to the optimal Sn content in the ITO film to induce a large number of free electron, causing a lower sheet resistivity. (C) 2019 The Electrochemical Society.