The iridium hydroxyoxide, IrOx(OH)(y), supported on antimony tin oxide was synthesized using a polyol method for oxygen evolution reaction. The effects of heat-treatment (calcination) on the chemical, physical and electrochemical characteristics of the electrocatalyst are studied. Changes in the physical and chemical structure of the electrocatalyst due to heat-treatment were investigated using X-ray diffraction, scanning electron microscopy/energy dispersive X-ray spectroscopy, transmission electron microscopy, nitrogen physisorption, and X-ray photoelectron spectroscopy. The electrocatalysts were also electrochemically characterized using cyclic and linear sweep voltammetry measurements. The results showed that the IrOx(OH) y electrocatalyst synthesized with the polyol method had an amorphous structure with a particle size distribution of 1-5 nm. Calcination at temperatures higher that 500 degrees C increased the particle size of the iridium species to approximately 30 nm, decreased the surface area by 50%, and altered the structure of iridium based materials from amorphous iridium-hydroxyoxide (IrOx(OH)(y)) to crystalline iridium oxide (IrO2). Furthermore, the oxidation charge measured from the cyclic voltammograms decreased by roughly 80% and the Tafel slope increased by 25%. The amorphous IrOx(OH)(y) electrocatalyst had higher activity and better performance (more than 90%) compared to the crystalline IrO2. (C) 2017 The Electrochemical Society. All rights reserved.