Soluble, monomeric Ir(III/IV) complexes strongly affect the photoelectrochemical performance of IrOx.nH(2)O-catalyzed photoanodes for the oxygen evolution reaction (OER). The synthesis of IrOx.nH(2)O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through monomeric intermediates that were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions, the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)(5)(H2O)](2) and [Ir(OH)(6)](2-), respectively. These monomeric anions strongly adsorb onto IrOx.nH(2)O colloids but can be removed by precipitation of the colloids with isopropanol. The monomeric anions strongly adsorb onto TiO2, and they promote the adsorption of ligand-free IrOx.nH(2)O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode redox cycle and thus dramatically degrades the photoelectrochemical performance of the cell. The growth of a dense TiO2 barrier layer prevents access of soluble monomeric anions to the interface between the oxide semiconductor and the electrode back contact (a fluorinated tin oxide transparent conductor) and leads to improved photoanode performance. Purified IrOx.nH(2)O colloids, which contain no adsorbed monomer, give improved performance at the same electrodes. These results explain earlier observations that IrOx.nH(2)O catalysts can dramatically degrade the performance of metal oxide photoanodes for the OER reaction.