In this paper, a Monte Carlo model is used to predict the light intensity profile inside a photocatalytic monolith reactor where the internal walls of the reactor are coated with a light-absorbing and light-reflecting catalytically active film. Reactors with channels of square, rectangular, equilateral triangular and isosceles triangular sections are considered. These cases have not cylindrical symmetry and a 2-D approach must be used. (c) 2005 Elsevier Ltd. All rights reserved.