The effect of pH in the impregnation process on the phase formation, structure and catalytic potential of Mo-Zn-Al-O catalysts synthesized by the co-precipitation and impregnation method was evaluated. The materials were characterized by X-ray diffraction, UV-vis diffuse reflectance spectroscopy, Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy and N-2 adsorption technique. The catalytic efficiency of Mo-Zn-Al-O catalysts was evaluated with respect to the degradation of cationic orchid X-BL by wet air oxidation under room conditions. The speciation diagram of Mo species in aqueous solution as a function of pH value was established. At basic pH (pH > 8), MoO42- was the main anionic in the solution. HnMo7O24m- was formed with the decrease of pH values ranging from 8 to 4 due to the combination of H+ and Mo7O246-. Molybdenum was present as solid of H2MoO4 when pH value was below 4.0. Interestingly, the Mo-Zn-Al-O catalyst prepared under pH = 7 has lower surface texturing values (SBEr = 9.9 m(2)/g and Vp = 0.083 cm(3)/g) than the Mo-Zn-Al-O catalyst prepared under pH = 4 (SBEr = 21.6 m(2)/g and Vp = 0.038 cm(3)/g). However, the Mo-Zn-Al-O catalysts prepared under pH = 7 shows the highest catalytic activity with 94.6% of decolorization and 86.7% of TUC removal efficiency. This highest catalytic activity can be contributed to special Mo species, special crystalline phases, tetrahedral Mo structure and highly dispersed Mo. (C) 2014 Elsevier B.V. All rights reserved.