The deactivation of Ir oxides supported on Al substrates has been studied in 0.5 M H2SO4. Their electrochemical behaviour and service life was also compared to IrO2 electrodes, similarly prepared, supported on Ti. The Ir oxides were prepared by thermal decomposition of an Ir salt precursor solution. The service life and other oxide properties were found to be influenced by different factors used for preparation of the Ir oxide electrodes, for example, the temperature used for the decomposition process and the solution used to etch the Al substrate. In contrast to the IrO2 anodes supported on Ti, the service lives of the IrO2 anodes supported on Al were found to be very short. The deactivation of the latter anodes appears to be related to poor adhesion between the Ir oxide and the Al substrate. However, it was found that the service life of IrO2 anodes supported on Al is increased when a layer consisting of iridium is electrochemically deposited onto the Al substrate prior to the thermal formation of the IrO2.