H3PO4 is commonly used as a proton-conducting phase in high temperature proton exchange membrane fuel cell membranes. However, its reduction with hydrogen at elevated temperatures yields compounds like H3PO3 and phosphorus. The aim of this work was to describe the basic electrochemical behaviour of H3PO3 and H3PO2 on a Pt electrode in diluted aqueous H2SO4 solutions. The results show that adsorption of both phosphorus acids studied becomes important at an oxoacid bulk concentration around and below 10 mol dm (3). Adsorption isotherms at 25 and 70 degrees C were determined for both acids. Unusually, the extent of adsorption increases with rising temperature. H3PO3 is anodically oxidised on a bare Pt as well as on a PtO surface. H3PO2 oxidation proceeds mainly on a PtO surface, with the intermediate product being H3PO3. High overvoltage around 1 V is characteristic of all anodic oxidation reactions occurring in the temperature range studied. (C) 2015 Elsevier Ltd. All rights reserved.