Heterogeneous photocatalysis with titanium dioxide TiO2 is an advanced oxidation processes that uses ultraviolet radiation to carry out reactions leading to the degradation of polluting agents in water. For the development and scaling up of efficient photocatalytic processes it is important to model the transport of radiation in the reactor volume, because it affects the photo reaction. In the present work we have used the P1 approximation of radiative transfer theory to evaluate radiation distributions inside a specific annular photocatalytic reactor known as Photo CREC–Water II. The reactor consists of two concentric tubes with a TiO2/water suspension flowing axially in the annular space between them, and a lamp at the center of the inner tube. The theoretical results are compared with experimental data from measurements of radiation transmission through several observation windows in the external reactor wall. In general the model describes the observed experimental results well. Due to the nature of the employed approximation a good quantitative agreement is possible only for suspensions with a large optical depth. For small optical depths the agreement is adequate only for the central portion of the reactor length.