The inhibiting properties of some organic phosphonium and ammonium compounds were studied with respect to the corrosion of zinc in 1 M H3PO4 solution. It was found that onium compounds which have pi-electron system(s) are adsorbed following Frumkin＇s adsorption isotherm and provide better inhibition efficiency than those containing no pi-electron system. The adsorption of the latter compounds was found to obey Langmuir＇s isotherm. Both potentiostatic and electrochemical impedance techniques proved that the studied onium compounds act as primary interface inhibitors without changing the mechanism of either hydrogen evolution reaction or zinc dissolution.