In this work, the mesoporous-assembled TiO2-NiO mixed oxide nanocrystals were successfully synthesized by a sal-gel process with the aid of a structure-directing surfactant under mild conditions. The photocatalytic activity of the synthesized mixed oxide nanocrystals for degradation of Methyl Orange (MO) azo dye used as a model wastewater contaminant was investigated under UV light irradiation. The synthesized mixed oxide nanocrystals were systematically characterized by TG-DTA, N-2 adsorption-desorption analysis, SEM-EDX, XRD, and UV-visible spectroscopy. The photocatalytic activity results showed that the incorporation of NiO phase with a suitable content into the TiO2-NiO mixed oxide increased the photocatalytic MO degradation efficiency. It was found that the mesoporous-assembled Ti75-Ni25 mixed oxide nanocrystal (with 75 mol % TiO2 and 25 mol % NiO) provided a maximum MO degradation efficiency as compared to the other investigated nanocrystals as well as the commercial P-25 TiO2 powder. The induced n-p junction at the interfacial contact between the TiO2 and NiO phases in the Ti75-Ni25 mixed oxide nanocrystal is supposed to efficiently prevent the electron-hole recombination upon the UV light excitation and consequently enhance its photocatalytic degradation activity. (C) 2012 Elsevier B.V. All rights reserved.