This work intends to develop a mathematical model of a fixed bed reactor and tests this model with experimental data. For this reason, we introduce some specific knowledge about light absorption from the catalyst, about the hydrodynamic interpretation of the reactor and the kinetic behaviour of photocatalytic reactions. Afterwards, we summarise this information and set up a complex mathematical model. We use this model in order to identify the kinetic constants of some reactions. We also formulate some simplified models to be used in particular circumstances and we compare the results of the complex model and the simplified models. Moreover, we compare experimental data of conversion and the results estimated with our model.