A computational fluid dynamics approach has been used to investigate the effect of lamp separation (X-lamp) on the radiation intensity distribution in a multiple-lamp photocatalytic reactor. The optical parameters (absorption and scattering coefficients) of Aeroxide (R) P25 titanium dioxide (TiO2) were determined by performing experiments using a single lamp system. Since the optical properties are wavelength dependent, the range of wavelength from the UV lamp was divided into 4 bands, and optical properties in each of the bands were determined by matching the experimental observations with simulated values. Simulations were then carried on multiple-lamp (2 and 4 lamps) photoreactors as a function of lamp separation and catalyst loadings. In case of 2-lamp system, the maximum local volumetric rate of energy absorption (< LVREA >) occurred at X-lamp=40 mm, and it was independent of the catalyst loading. With 4 lamps however, optimum X-lamp was dependent on the catalyst loading. At low loads (up to W-cat=0.06 gL(-1)), the optimum X-lamp was 80 mm but as the catalyst concentration increased, the value of the optimum lamp separation decreased considerably, with 30 mm for W-cat=0.07 gL(-1) and decreasing further as the concentration further increased. Because of the high absorption coefficient of the catalyst, the wall emissivity had a negligible effect on the < LVREA > for both configurations, even when the lamps were close to the wall. Finally, in both cases, the optimum lamp separation was independent of the lamp emissive power. (C) 2013 Elsevier Ltd. All rights reserved.