The global process of heterogeneous photocatalytic oxidation can be described in consecutive steps of mass transfer, adsorption, reaction and desorption. Although such a deseription normally only considers the reactions in the aqueous solution, some compounds are degraded by photogenerated holes in the surface of the solid. In this work, the photocatalytic degradation of a leather dye, Direct Black 38, was studied as a model compound to elucidate the steps during the photocatalytic degradation. The mass transfer coefficients were evaluated through the description of the adsorption kinetics using the film and pore diffusion model. The equilibrium of adsorption was described according to the Langmuir model. Under irradiation, the degradation of the dye was measured and a pseudo-first order kinetic law was used to depict the reaction. The Weiz-Prater criterion was used to determine the rate controlling step of the reaction with the result that no diffusion limitations for the reaction existed. The characterization of the solid after adsorption and after desorption showed the presence of the reaction's intermediate species.