Effects of temperature and pH of the plating baths, potential ranges and scan rates of cyclic voltammetry, and concentrations of RuCl 3 .xH 2 O on the deposition rate and the physicochemical properties of hydrous ruthenium oxide (i.e. RuO y .nH 2 O) are investigated systematically. For the RuO y .nH 2 O deposition, the apparent reaction order of RuCl 3 .xH 2 O is 1 while effects of pH are very complicated, probably due to the influence on the structure of the dissolved chloro-hydroxyl-ruthenium species, rendering a change in deposition mechanism. The redox transitions of oxyruthenium species between different oxidation states within the RuO y .nH 2 O deposits are electrochemically reversible while the electrodes covered with a very thin RuO y .nH 2 O film exhibit a less reversible characteristics. Surface morphologies and crystalline information of these RuO y .nH 2 O deposits are examined by scanning electron microscopy and X-ray diffraction, respectively.