The radiation field in an annular, slurry photocatalytic reactor has been modeled and verified working with a pilot-plant reactor. A suspension of titanium dioxide in water flows through the annular reaction space, which is irradiated with a tubular, UV lamp placed at the central axis of the system. Two different types of titanium dioxide have been used: Aldrich and Degussa P25. Using a rigorous model, it is possible to precisely evaluate the radiation intensity at each point inside the reactor. The radiation distribution was verified by computing the forwardly transmitted radiative fluxes going out of the reactor walls at different axial positions and compared with radiometer measurements ("point" measurements). Predictions of the same fluxes were also verified with homogeneous actinometry employing potassium ferrioxalate (providing an average measurement of the outgoing fluxes over the reactor external surface). When predictions from the theoretical model are compared with experimental data, good agreement was obtained.