A study of the anodic impurity peak that appears in the cyclic voltammograms of platinum electrodes in highly purified, hot, concentrated phosphoric acid is reported. The results indicate that this anodic impurity peak corresponds to the anodic oxidation of phosphorous acid. This phosphorous acid impurity appears to be formed at cathodic potentials by the reaction of the phosphoric acid electrolyte with adsorbed hydrogen. The electrochemical formation and anodic oxidation of the phosphorous acid impurity are enhanced at the elevated temperatures found in phosphoric acid fuel cells.