Understanding what controls the reaction rate on iridium-based catalysts is central to designing better electrocatalysts for the water oxidation reaction in proton exchange membrane electrolysers. Here we quantify the densities of redox-active centres and probe their binding strengths on amorphous IrOx and rutile IrO2 using operando time-resolved optical spectroscopy. We establish a quantitative experimental correlation between the intrinsic reaction rate and the active-state energetics. We find that adsorbed oxygen species, *O, formed at water oxidation potentials, exhibit repulsive adsorbate–adsorbate interactions. Increasing their coverage weakens their binding, thereby promoting O–O bond formation, which is the rate-determining step. These analyses suggest that although amorphous IrOx exhibits a higher geometric current density, the intrinsic reaction rates per active state on IrOx and IrO2 are comparable at given potentials. Finally, we present a modified volcano plot that elucidates how the intrinsic water oxidation kinetics can be increased by optimizing both the binding energy and the interaction strength between the catalytically active states.