The adsorption of H2PO4- ions was studied on low Miller index rhodium, single crystal electrodes by in situ FTIR spectroscopy. It is found that for Rh(100) and Rh(110), H2PO4- ions are the major species at low potentials, but at higher potentials, some of the H2PO4- ions undergo a potential induced deprotonation and probably there is a mixture of H2PO42- and HPO42- ions. On Rh(111) the deprotonation starts at very low potentials and at higher potentials the H2PO4- is fully converted to HPO42- . The behavior of the band center and of the band intensity with the applied potential was also analyzed. It is found that the adsorption increases from 0.08 V vs. a Pd-H-2 electrode up to 0.5 V and then it decreases when the OH starts to be adsorbed. (C) 2003 Published by Elsevier B.V.