The a.c. impedance response of sputtered iridium oxide films (SIROFs) was studied at room temperature in 1 M H2SO4 between 1 mHz and 50 mHz. The spectra were recorded as a function of applied potential in the range of electrochromic properties from 0.0 to 1.0 V vs SCE and before and after an electrochemical treatment consisting of alternatively colouring and bleaching the electrode. The spectra were analysed with help of an equivalent circuit. Between 0.4 and 1.0 V, the spectra can be interpreted as due to electrochemical proton insertion in a single phased compound. From the data, hydrogen chemical diffusion coefficients with values ranging from 2 x 10(-8) to 1.1 x 10(-7) cm(2) s(-1) are found. It is shown that this parameter increases fourfold after the cycling treatment and significantly decreases with the amount of inserted hydrogen. Below 0.4 V spectrum changes are observed over the intermediate frequency range studied, indicating some changes of the interfacial reactivity which remain to be clarified.