Light-driven decomposition of Ir(acac)₃ spin-cast on a conducting glass substrate produces a thin conformal film of amorphous iridium oxide, a-IrO_x. The decomposition process, which was carried out under an ambient atmosphere at room temperature and tracked by Fourier transform infrared (FTIR) spectroscopy, appears to proceed by way of a ligand-to-metal charge transfer (LMCT) process. The amorphous nature of the films is based on the lack of any observable Bragg reflections by powder X-ray diffraction techniques; the elemental composition was corroborated by X-ray photoelectron spectroscopy (XPS) measurements. The films are found to be excellent electrocatalysts for mediating the oxygen evolution reaction (OER) in acidic media, as evidenced by the onset of catalysis at 130 mV and a Tafel slope of 34 mV dec^–1. These parameters enable current densities of 1 and 10 mA cm^–2 to be reached at 190 and 220 mV, respectively. Exposing the films to higher temperatures (500 °C) renders a film of crystalline iridium oxide, c-IrO_x, which displays a Tafel slope of 60 mV dec^–1, thus requiring an additional 50 mV to reach a current density of 1 mA cm^–2. The film of a-IrO_x reported here is among the best OER electrocatalysts reported to date.