Ruthenium, iridium, and mixed ruthenium/iridium oxide layers on titanium substrates have been obtained by thermal decomposition of chloride solutions. The decomposition temperature of RuO2 and IrO2 was varied from 300 to 500 degrees C and from 400 to 500 degrees C, respectively. That on the mixed RuxIr1-xO2 layer was kept constant at 400 degrees C. For the mixed oxide electrode, the Ru content was varied over the whole compositional range. Current-potential curves and cyclic voltammetry measurements were performed in 1 M H2SO4. It is shown that such oxide layers can be activated through cathodic polarization, leading to an increase of the electrocatalytic activity for hydrogen evolution. The ratio between the current density at a given electrode potential before and after completion of the activation process or the ratio between the exchange current density before and after the completion of the activation process was used to quantify this activation phenomenon. Values as high as 100 have been observed in some cases, but typical values are around ten. Through a series of specific measurements and comparison with data taken from the literature, it is shown that this activation phenomenon is not related to an increase of the electrochemically active surface area as determined through cyclic voltammetry measurements. An explanation is proposed whereby H-chemisorption within the oxide layer is ultimately responsible for the increase of the electrocatalytic activity of the oxide layer.