In this work, the degradation of tetrachloroethylene or perchloroethylene WE) in an air stream was studied for different values of PCE feed concentrations, relative humidity levels, and light intensities. The TiO2 photocatalyst was deposited as a thin film on a borosilicate glass plate by means of a sol-gel technique. The reactor was operated under kinetic control regime. An expression of the intrinsic reaction kinetics was developed. This expression is based on a model of the absorption of radiant energy on the surface of the catalyst, followed by a detailed reaction mechanism based on a reaction scheme that involves Cl-center dot as an active reaction intermediate. Experimental results from a differential photoreactor were used to validate the proposed kinetic model and to determine the kinetic parameters by nonlinear regression of experimental data. The obtained results show: (1) first-order kinetics with respect to the PCE concentration, (2) linear dependence with the photon absorption rate, and (3) site-competitive adsorption between PCE and water, resulting in a dependence of the kinetics on the relative humidity.