Abstract. In this work, a novel phosphate ion-selective sensor based on a cobalt thin-film modified ionic liquid-carbon paste electrode was developed. This sensing electrode was prepared by electrodeposition of cobalt nanoparticles onto the surface of an ionic liquid-carbon paste electrode. Cyclic voltammetry and scanning electron microscopy were employed to characterize the cobalt film. With the unique properties of the cobalt nanostructure film, the developed electrode exhibited very attractive potentiometric characteristics for phosphate analysis. Several influencing parameters were investigated in detail, including dissolved oxygen and solution stirring rate. The developed electrode showed a wide linear range from 5 A— 10 -6 to 1 A— 10 -1 mol L -1 with a response slope of -55.2 mV decade -1 . The detection limit of the electrode for phosphate was 1 A— 10 -6 mol L -1 . Furthermore, the proposed electrode was applied to determine phosphate in environmental water samples. The detection results were in good agreement with those obtained by spectrophotometry. Dissolved oxygen exhibited no obvious effects on real sample analysis. The results suggested that the cobalt thin-film modified ionic liquid-carbon paste electrode was sensitive, reliable, and effective for the potentiometric determination of inorganic phosphate.