The highly dispersed nature of hydrous iridium oxide combined with its electrochemical properties makes it a very interesting material. Possible applications can be found in electrocatalysis, neural stimulation, electrochromic devices and pH sensors. In the present work a commonly used electrodeposition solution, based on IrCl4, oxalic acid, H2O2 and NaCO3, was studied with electrochemical methods as well as UV-vis spectroscopy. The hexachloroiridate (IV) complex was initially observed in both UV-vis and cyclic voltammetry. No oxalato complexes were detected, instead oxalate is proposed to act as a stabilising agent in nanoparticle formation. Initially hydrogen peroxide was found to reduce Ir(IV) complexes to Ir(111). However, after increasing the pH by addition of sodium carbonate it was shown to act as an oxidising agent instead. During development of the solution UV-vis showed the formation of multinuclear complexes and with aging also scattering from solid materials was observed. Transmission electron microscopy confirmed the formation of nanoparticles of iridium oxide with a diameter of similar to 3 nm. The role of nanoparticles and non-particulate species in the deposition process is discussed. (C) 2012 Elsevier Ltd. All rights reserved.