BACKGROUND: Owing to difficulties in catalyst recovery from slurries the deposition of a catalyst on suitable supports has been investigated extensively in recent years. The support material and the method of coating influence the activity. Photocatalytic decolorization of methyl orange (MO) and Rem Red F3B (RRF3B) was conducted in various reactor systems including a slurry reactor, ZnO thin film coated tube reactor and fixed bed reactor filled with ZnO coated ceramic or glass supports of various geometries. RESULTS: ZnO coating was carried out by the ammonium zincate deposition method. ZnO thin films were quite stable in acidic and basic media and resistant to photocorrosion. Various methods including scanning electron microscopy (SEM) and X-ray diffraction (XRD) were applied for the physical characterization of thin films. The XRD patterns of ZnO thin film exhibit a wurtzite (zincite) crystal structure. SEM analysis revealed the granular morphology of ZnO film with a particle size of 300-400 nm. CONCLUSION: Photocatalytic activities were determined kinetically by calculating first-order rate constants, which were also related to process variables by regression analysis. From the viewpoint of decolorization efficiency, the coated tube reactor (6 mm i.d.) showed the highest activity, which was equal to that of a powder catalyst loading of 300 mg dm(-3) for MO and of 200 mg dm(-3) for RRF3B. (C) 2010 Society of Chemical Industry