A first-principles mathematical model describes performance of a titania-coated hon-( ) eycomb monolith photocatalytic oxidation PCO reactor for air purification. The single-channel, 3-D con®ection᎐ diffusion᎐ reaction model assumes steady-state operation, negligible axial dispersion, and negligible homogeneous reaction. The reactor model accounts rigorously for entrance effects arising from the de®eloping fluid-flow field and uses a pre®iously de®eloped first-principles radiation-field submodel for the UV flux profile down the monolith length. The model requires specification of an intrinsic photocatalytic reaction rate dependent on local UV light intensity and local reactant concentration, and uses reaction-rate expressions and kinetic parameters determined independently using a flat-plate reactor. Model predictions matched experimental pilot-scale formaldehyde con®ersion measurements for a range of inlet formaldehyde concentrations, air humidity le®els, monolith lengths, and for ®arious monolithrlamp-bank configurations. This agreement was realized without benefit of any adjustable photocatalytic reactor model parameters, radiation-field submodel parameters, or kinetic submodel parameters. The model tends to systematically o®erpredict toluene con®ersion data by about 33%, which falls within the accepted limits of experimental kinetic parameter accuracy. With further ®alidation, the model could be used in PCO reactor design and to de®elop quantitati®e energy utilization metrics.