The radiation field in an annular photocatalytic reactor is simulated using a Monte Carlo method (MC) for two TiO2 suspensions in water. Simulations are performed by using both the spectral distribution and the wavelength-averaged scattering and absorption coefficients. The Henyey-Greenstein phase function is adopted to represent forward, isotropic, and backward scattering modes. It is assumed that the UV lamp reflects the backscattered photons by the slurred medium. Photoabsorption rates using MC simulations and spectral distribution of the optical coefficients agree closely with experimental observations from a macroscopic balance. It is found that the scattering mode of the probability density function is not a critical factor for a consistent representation of the radiation field. MC simulation for the optimal catalyst concentration reveals that the maximum LVREA is reached at a concentration of 0.14 g L-1 for TiO2 Degussa P25. From this concentration, the apparent optical thickness is determined to be 2.8476 which is in agreement with the optimal one previously reported. This concentration is comparable to that determined experimentally for phenol photocatalytic degradation.