The surface activation of ruthenium oxide electrodes under cathodic polarization and in conditions where hydrogen evolution occurs is investigated in three different series of experiments. The first series deals with real-time AFM imaging performed on a thin RuO2 layer on Ti substrates prepared by thermal decomposition. in the second series, ex situ AFM observations are performed on an RuO2 electrode after cathodic treatment at a higher current density than that accessible through the use of the electrochemical AFM. Finally, the third series of measurements relates to the ex situ STM examination of an RuO2 single crystal before and after hydrogen evolution. In each case it was shown that permanent surface deformation occurs on a timescale comparable with that required for the activation of the electrode to proceed. At the end of the activation process, the surface of the electrode is rougher. This behavior is also observed on an RuO2 single crystal electrode, and is thus an intrinsic property of the oxide material. An explanation is given, in which the chemisorption of hydrogen within the structure of the oxide material plays a central role.