The treatment of Direct Yellow 86 dye wastewater by the UV/H2O2 process in continuous annular photoreactors was studied under various UV light intensities, influx concentrations of dye, dosages of H2O2 and dimensions of photoreactor. A photoreactor design equation combined the UV light distribution profile in the reactor and the empirical rate expressions for the decomposition of dye and H,0, was used to predict the destruction of dye within photoreactors of different geometries at various operating conditions. Experimentally observed removal of the dye pollutant in the plug flow annular reactor agreed well with the theoretical solutions modeled by the developed photoreactor design equation. (C) 2002 Elsevier Science B.V. All rights reserved.