The photoactivity of metal-oxide nanoparticles can be elevated and extended in the visible range through doping, sensitization, nanostructuration, and phase mixing. The present work reports synthesis of highly photoactive cobalt-doped mixed phase/biphasic zinc oxide nanoparticles with zinc-blende and wurtzite phases. The photoactivity of the nanoparticles, assessed through photocatalytic degradation kinetics of methylene blue and phenol, shows over 3 and 1.5 times more visible light photoactivity than monophasic (wurtzite) undoped ZnO and doped Co-ZnO counterparts, respectively. The increase in the activity is mainly attributed to red-shift in the absorption spectrum due to doping. But in the mixed phase (biphasic) Co-ZnO it is further complemented by the synergy of the interacting phases at the homojunction and the increase in oxygen vacancies at the surface which ensure availability of more photogenerated carriers by contributing in their generation and/or separation. The synthesis of the novel mixed-phase photocatalyst has been done through a sol-gel based facile and single-step process wherein cobalt plays the unique role of dopant-cum-substrate. (C) 2013 Elsevier B.V. All rights reserved.