Potential dependent changes in pH were observed at the platinum/solution interface by monitoring changes in the equilibrium concentration of phosphates using infrared spectroscopy. The pH at the platinum/solution interface was observed to decrease with increasing positive potentials in alkaline solutions. At potentials in the oxide region of platinum, changes in the composition of bulk solution arise owing to the formation of surface oxides. The potential dependent pH changes observed in these studies are consistent with theories concerning the interfacial properties of hydrogen adsorbing metals. Such potential dependent pH changes may have important consequences when studying the electrochemical reactivity of compounds which are sensitive to pH.