Nanocrystalline zinc ferrite (ZnFe2O4) photocatalysts with different crystallite sizes prepared using the colloid mill and hydrothermal technique are reported. This synthetic approach involves a very rapid mixing of Fe3+ cations with reducing agent and reduction process in a colloid mill reactor, followed by a slow oxidation of iron nuclei and structural transformation in a separate hydrothermal process. Material characterization has been presented by powder X-ray diffraction (XRD), chemical element analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectra. The results indicate that ZnFe2O4 nanocrystals with the uniform crystallite sizes have been obtained using in situ forming iron nuclei as the source of Fe. A possible formation mechanism of ZnFe2O4 nanocrystals was proposed. Furthermore, through photocatalytic investigation, these ZnFe2O4 nanocrystals displayed better abilities to photodecompose acid orange II azodye molecule under UV irradiation due to quantum confinement effect and high surface area structure, as compared to bulk ZnFe2O4 sample prepared by the conventional solid-state method. Since as-synthesized ZnFe2O4 nanocrystals have excellent chemical and thermal stabilities and exhibit good photocatalytic activities, it can be expected that they may have potential application in the field of industrial photo-degradation of organic azodye pollutants. (C) 2009 Elsevier B. V. All rights reserved.