Nanocrystalline Mg-doped ruthenium dioxide catalysts with the formula Ru1-xMgxO2 (0 <= x <= 20) were synthesized by the sprayfreezing freeze-drying technique. Synthesized materials are of nanoparticulate nature and show a single phase diffraction pattern conforming to a tetragonal oxide of the rutile structural type. Magnesium ions are not distributed homogeneously in the material, but exist in Mg-rich clusters as shown by X-ray absorption spectroscopy. The refinement of the Mg EXAFS functions for materials with low Mg content shows that the magnesium rich clusters contain Mg in a highly strained environment similar to that of the rutile-type structure. The Mg environment shifts to an ilmenite-type inclusion when Mg occupies more than 10% of all cationic positions. All Mg modified materials are active in oxygen evolution and chlorine evolution reactions. Although the Mg containing catalysts show lower overall activities compared with the non-doped ruthenia, they feature enhanced selectivity toward the chlorine evolution process, which is attributed primarily to the opening of a reaction pathway for chlorine evolution associated with presence of Mg modified active sites. (C) 2014 The Electrochemical Society. All rights reserved.