A reflection-type electrochemical cell was used to perform electrochemical in situ X-ray diffraction studies of RuO2 and IrO2 electrodes. X-ray diffractograms were recorded in situ as a function of the polarization time while the electrodes were poised at -0.5 V vs SCE in 1 M H2SO4 at room temperature. At this potential, strong hydrogen evolution is observed at the surface of the electrodes. In both cases, there is a shift of the X-ray diffraction peaks of the rutile structure towards the lower 20 angle values with the polarization time, indicating that the volume of the unit cell increases. In the case of RuO2, the unit cell volume increases from 64.00 to 65.34 Angstrom(3) which corresponds to a volume increase of similar to2%. In comparison, the unit cell volume of IrO2 increases from 65.56 to 66.00 Angstrom(3) (0.66%). These changes in the structure of the oxides occur on the time scale of a few hours. These modifications are not totally reversible, as the volume of the unit cell of both RuO2 and IrO2 stays slightly expanded compared to that of the pristine materials when the electrodes are brought back to open circuit conditions after extensive cathodic polarization. In the case of RuO2, the expansion of the unit cell occurs almost exclusively along the a-a plane, suggesting that hydrogen is inserted between the O-O atoms in this plane. The significance of these results in the context of the activation process of RuO2 and IrO2, that is the propensity of these oxides to become better performing electrode materials when they are cathodically polarized, is discussed. (C) 2004 Elsevier B.V. All rights reserved.