Lanthanide (Ln = La, Eu, Gd, Dy and Ho) loaded ZnO catalysts (Ln-ZnO) were synthesized, characterized and tested for the decolorization of methyl orange (MO). X-ray diffraction (XRD) analysis yields crystalline sizes of ZnO and Ln(2)O(3) nanoparticles. Surface area (BET) measurements showed higher surface areas and pore volumes for Ln-ZnO catalysts in comparison to bare ZnO. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images revealed obvious variations in the surface morphology of the bare catalyst after Ln loading. Energy dispersive X-ray (EDX) analysis and elemental mapping images displayed the existence of Ln ions on Ln-ZnO catalysts. X-ray photoelectron spectroscopy (XPS) confirmed the formation of ZnO nanoparticles while the form of Zn2+ oxidation state on the catalyst surfaces was demonstrated by the sharp Zn 2p peaks. The absorption edges in the UV-Vis diffuse reflectance spectra (UV-Vis DRS) of all Ln-ZnO catalysts were found to be slightly shifted to higher energy regions. The photocatalytic performances of the bare ZnO and Ln-ZnO catalysts were evaluated under UV irradiation following pseudo-first order kinetics. The highest normalized rates were obtained in the presence of the bare catalyst. Among the Ln-ZnO catalysts, 1% La-ZnO demonstrated the highest activities. The photocatalytic reactions were also explored in terms of Langmuir-Hinshelwood model. Ln ions were discussed in terms of their loading percentages, complex formation abilities with Lewis bases, electronic configurations, adsorption capacities, electron accepting and hydroxyl radical (OH) formation capabilities. A plausible mechanism was proposed for the route of MO degradation. (C) 2014 Elsevier B.V. All rights reserved.