Dedicated to Professor Andre¬ M. Braun on the occasion of his 60th birthday A model for scaling up a homogeneous photoreactor was developed and experimentally verified in a pilotplant-size apparatus. The procedure is exemplified by the oxidation of dilute aqueous HCOOH solutions with UV radiation (254 nm) and H 2 O 2 . First, the kinetic model and the kinetic parameters of the HCOOH degradation were obtained in a well-stirred, small, batch flat-plate photoreactor (volume 70 ml). The method employed in the analysis of the experimental results yielded reaction-rate expressions for HCOOH and H 2 O 2 that were independent of the reactor configuration. These kinetic equations and the corresponding kinetic constants were then used in a mathematical, fully deterministic model of a continuous-flow, 2-m-long, annular reactor (0.0065 m 2 of cross section for flow) operating in a laminar-flow regime to predict exit concentrations of HCOOH. Irradiation was provided in both cases by two different types of germicidal lamps. No additional experiments were made to adjust the reactor-model parameters. Theoretical predictions from the representation of the reactor performance obtained were compared with experimental data furnished by experiments in the much-larger-size, cylindrical-flow reactor. Results showed good agreement for the range of variables explored; they corresponded to expected operating conditions in water streams polluted with low concentrations of organic compounds.