Mg1-xNixO solid solution powder samples with different chemical compositions were prepared by heating MgO-NiO mixtures at 1300 degrees C for 12 h in air. From XRD measurement, all powder samples were indexed as a single phase of cubic structure, of which the diffraction peaks shifted to high-angle side with the increase of doping amount of NiO. The pH values of the solution dispersed with the powder samples decreased when the doping amount of NiO in solid solution was increased. Antibacterial activity of the powder samples was examined by colony count method. In the result, the antibacterial activity of Mg1-xNixO was remarkably weaker than original MgO powders, irrespective of the kind of bacteria. In addition, it was found that the antibacterial activity of Mg1-xNixO reduced with increasing the doping amount of NiO. Two factors, the generated amount of O-2(-) and the eluted amount of Ni2+ ions affected the antibacterial activity of Mg1-xNixO solid solution. Especially, the stability of O-2(-) in aqueous solution is dependent on pH value. Therefore, the strength of antibacterial activity was associated with the pH values in the dispersed solution of Mg1-xNixO. (C) 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.