For reasons of electrode integrity and durability it is desirable to add zirconium hydrogen phosphate to electrocatalytic layers in PEM water electrolysis. Common synthesis methods are frequently associated with an accompanying addition of phosphoric acid. The presence of free phosphoric acid is known to be detrimental for catalytic activity of the common oxygen evolution catalysts. In this work composite electrocatalytic layers of iridium oxide, Nafion (R) and zirconium hydrogen phosphate were fabricated on glassy carbon disk electrodes. Zirconium hydrogen phosphate was prepared by mixing zirconium oxychloride and phosphoric acid. The effect of the electrocatalyst constituents were electrochemically investigated with respect to the oxygen evolution reaction in 0.5 M sulfuric acid electrolyte at room temperature. Slow linear sweep voltammograms showed that the electrocatalyst performance depends only to a small extent on the way the composite electrodes were manufactured. However, the coexistence of zirconium hydrogen phosphate particles in the electrode structure offsets the destructive effect of any phosphoric acid remaining in the electrode more or less completely. Inherent catalyst activity is unaffected by the addition of zirconium hydrogen phosphate, but apparently worsen the current per mass emphasizing the necessity of optimizing the catalytic layers with respect to performance and mechanical properties. (C) Copyright 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.