A kinetic model for the photocatalytic degradation of formic acid in an immobilized system is presented, including the dependency of the reaction rate on the concentration of formic acid and oxygen, the catalyst layer thickness and the light flux. In the system some external mass transfer limitation occurs which is included in the modeling with experimentally determined values for the mass transfer coefficient of both formic acid and oxygen. The model describes the measurements well. The degradation rate appears to depend linearly on the light intensity. The adsorption of formic acid and oxygen on the catalyst layer appears to play an important role in the degradation rate.