A two-dimensional (2-D) mathematical model to describe the photocatalytic degradation of aqueous-phase organic compounds in a multichannel optical fiber monolith reactor (OFMR) was developed and validated by experimental results. The model incorporated an empirical radiation field submodel, an annular flow dynamics submodel, and a Langmuir-Hinshelwood kinetics submodel. Parameters for the radiation field submodel were derived experimentally. The rate constant in the model accounted for both surface adsorption rate and intrinsic reaction rate. The model presented may be used for the design and optimization of the OFMR. Satisfactory agreement between the model predictions and experimental results over a range of experimental conditions was observed. The effects of the reactor geometry including the fiber diameter, the ratio of the fiber diameter, and the channel diameter and the reactor length were investigated. Higher efficiency of degradation was observed for the reactor with the larger fiber and smaller ratio of the fiber diameter to the channel diameter at the expense of a lower liquid throughput. The reactor efficiency was higher in the upflow mode than in the downflow mode for the water stream. (c) 2006 American Institute of Chemical Engineers