Multilayer hydrous oxide formation and reduction on platinum in aqueous phosphoric acid solution was investigated using potentiodynamic techniques. The behavior of this system was rather similar to that reported earlier for platinum in aqueous sulfuric acid. However, it was noted that such deposits could be produced under quite severe conditions, e.g., 95% H3PO4 at 160-degrees-C; thus they may be of relevance to the operation of the oxygen cathode of phosphoric acid fuel cells. Evidence of unusual, independently reported, premonolayer oxidation responses (within the double layer region) for the platinum/H3PO4(aq) interface was confirmed. Since the role of impurities in this area has been discounted previously, and the effects may also be observed with H2SO4 as electrolyte, these premonolayer peaks are attributed to formation of active oxide species. There is convincing evidence for the latter phenomenon in the case of gold in aqueous media. Premonolayer oxidation may inhibit the performance of the oxygen cathode in a fuel cell and a means of minimizing this effect may yield a significant increase in energy conversion efficiency.