Effects of temperature and pH of the plating baths, potential ranges and scan rates of cyclic voltammetry, and concentrations of RuCl3·xH2O on the deposition rate and the physicochemical properties of hydrous ruthenium oxide (i.e. RuO y ·nH2O) are investigated systematically. For the RuO y ·nH2O deposition, the apparent reaction order of RuCl3·xH2O 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 ·nH2O deposits are electrochemically reversible while the electrodes covered with a very thin RuO y ·nH2O film exhibit a less reversible characteristics. Surface morphologies and crystalline information of these RuO y ·nH2O deposits are examined by scanning electron microscopy and X-ray diffraction, respectively.