Nano-particulate Co doped ruthenium dioxide electrocatalysts of the general formula Ru1-xCoxO2-y (0 < x 0.3) were prepared by a co-precipitation method. The electrocatalysts with x < 0.2 conform to a single phase nano-crystalline materials. On the local level the Co forms clusters dispersed in the original rutile-like matrix. The local environment of the Co conforms to a rutile model which preserves the cationic arrangement but suppresses the probability of the Ru-Ru and Co-Co neighbors along the shortest metal-metal bonds. The electrocatalytic activity of the synthesized Ru1-xCoxO2-y materials in oxygen evolution is comparable with that of the non-doped ruthenium dioxide and little depends on the actual Co content. In presence of chlorides the Co doped materials are more selective towards oxygen evolution compared with the non doped ruthenia. The enhanced oxygen evolution in the case of Co doped electrocatalysts can be attributed to a chemical recombination of surface confined oxo-species. The selectivity shift towards oxygen evolution can be linked with limited activity of the Ru1-xCoxO2-y materials in the chlorine evolution reaction which seems to be relatively weakly dependent on the chloride concentration. (C) 2012 Elsevier B.V. All rights reserved.