IrO2 is a state-of-the-art catalyst that can be used in acidic conditions and has potential for application as an oxygen evolution reaction (OER) catalyst in proton exchange membrane electrolyzers. IrO2 is subjected to heat treatment at the synthesis and at the preparation of the catalyst layer, and fundamental studies regarding the synthesis conditions, resulting surface properties, and crystallinity with respect to the heat treatment are therefore of great importance. Herein, we prepared IrO2 thin films annealed at different temperatures and investigated the relationship among the OER activity, crystallinity, film resistivity, and annealing temperature using various analytical techniques. We show that the areal OER activity decreases as the annealing temperature increases, but the specific OER activity remains unchanged. There was no correlation between the specific OER activity and film properties such as conductivity. On the other hand, a correlation was observed between the electrochemical active surface area and Ir 4f peak position, which changed with the annealing temperature of the prepared IrO2 films. Graphic