A new transparent iridium oxide (IrOx) film on fluorine-doped tin oxide (FTO) electrodes were achieved from a homogeneous precursor complex solution by employing a facile spin-coating technique. The composition of the nanostructure and crystallinity of the IrOx film is tunable by a simple annealing treatment of a compact complex layer, which is responsible for their significantly different electrocatalytic performances for water oxidation. Transmission electron microscopy (TEM) observations showed uniformly dispersed small IrOx nanoparticles of dimensions ca. 2-5 nm for the film annealed at 300 degrees C, and the nanoparticles gradually agglomerated to form relatively large particles at higher temperatures (400 and 500 degrees C). The IrOx films prepared at different annealing temperatures are characterized by Raman spectroscopic data to reveal intermediate IrOx(OH)(y) nanoparticles with two oxygen binding motifs: terminal hydroxo and bridging oxo at 300 and 350 degrees C annealing, via amorphous IrOx at 400 degrees C, transforming ultimately to crystalline IrO2 nanoparticles at 500 degrees C. Cyclic voltammetry suggests that the intrinsic activity of catalytic Ir sites in intermediate IrOx(OH)(y) nanoparticles formed at 300 degrees C annealing is higher in comparison with amorphous and crystalline IrOx nanoparticles. Electrochemical impedance data showed that the charge transfer resistance (R-ct = 232 Omega) for the IrOx(OH)(y) film annealed at 300 degrees C is lower relative to that of films annealed at higher temperatures. This is ascribable to the facilitated electron transfer in grain boundaries between smaller IrOx particles to lead the efficient electron transport in the film. The high intrinsic activity of catalytic Ir sites and efficient electron transport are responsible for the high electrocatalytic performance observed for the intermediate IrOx(OH)(y) film annealed at 300 degrees C; it provides the lowest overpotential (eta) of 0.24 V and Tafel slope of 42 mV dec(-1) for water oxidation at neutral pH, which are comparable with values for amorphous IrOx center dot nH(2)O nanoparticle films (40-50 mV dec(-1)) reported as some of the most efficient electrocatalysts so far.