In this work, we developed thick film pH sensors based on the nanostructured RuO2-SnO2 system and analyzed their sensing mechanism using the potentiometric and electrochemical impedance spectroscopic (EIS) methods. The crystal structure and morphology of the sensitive films were investigated by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. The potentiometric sensor exhibits Nemstian behavior (56.5 mV/pH) in the pH range of 2-12. It shows very fast response (5 sand 9 s, for acidic to basic and basic to acidic solutions), long lifetime, small hysteresis effect, and good reproducibility. Selectivity test proved that the presence of Li+, Na+, and K+ ions in a solution had no significant influence on the sensor performance. Conductance, capacitance and impedance of the conductimetric pH sensor with the interdigitated electrode were found to vary with changing pH and as a function of the applied frequency in the range of 10 Hz-2 MHz. The EIS analysis of the complex impedance data by using Nyquist and Bode plots revealed the information about the underlying processes, like adsorption, diffusion and charge transfer, occurring at the metal oxide-solution interface. The proposed thick film pH sensors were intended to be used for water pollution monitoring. (C) 2015 Elsevier B.V. All rights reserved.