This study analyses issues and limitations regarding the definition of the local volumetric rate of photon absorption (LVRPA) in an annular Photo-CREC Water II reactor. This analysis is carried out in order to establish the influence of the scattering phase functions and boundary conditions (BC) on the LVRPA. To) accomplish this, macroscopic radiation quantities such as the total rate of photon absorption (TRPA) and the total transmitted radiation (TT) as functions of pholocatalyst concentration are experimentally determined. On the other hand, the radiative transfer equation (RTE) is solved using a Monte Carlo method (MC). Boundary conditions accounting for lamp absorption/re-emission effects and diffuse reflection/absorption at the inner and outer reactor walls are employed. The Henyey-Greenstein and the binomial phase functions are used to simulate both forward and backward scattering phase functions. The significant influence of the phase functions on the radiation field is assessed for various BCs. Simulation results show that in annular photo reactors, the sensitivity of the LVRPA towards the "g" scattering parameter increases when "g" is set in the forward scattering range. Moreover, the comparison with experimental macroscopic quantities proves that Degussa P25 displays mostly forward scattering. The investigation of various possible BCs also proves that TT and TRPA fitting yields scattering parameters in restricted ranges. Consideration of the more physically sound BCs applicable in the Photo-CREC Water II, with complete absorption in the outer wall leads to a "g" value in the 0.6-0.8 range. It is proven that the MC simulation and the use of TT measurements in the PhoLo-CREC-Water II reactor with selected BCs are of critical importance for establishing phase functions and scattering parameters in photo catalytic reactors. (C) 2013 Elsevier Ltd. All rights reserved.