The kinetics study of the adsorption and degradation phenomena involved in the photocatalytic degradation of Methyl Orange (MeO) were investigated using a batch reactor and artificial UV-light. Experiments were performed in a suspended TiO2 (Degussa P-25) system at pH = 3 and mass catalyst 0.8 g/L. The initial concentrations of MeO were varied in the range 5-75 ppm. From the results, the adsorption was found to be an essential factor in the photodegradability of the dye. The Linear transform of the Langmuir isotherm curve was further used to determine the characteristic parameters witch were: Q(ma) = 14.65 mg g(-1) and K-ad = 7.06 x 10(-2) L mg(-1). The kinetic analysis of the photodecomposition of MeO showed that the disappearance followed satisfactory the pseudo first-order according to Langmuir-Hinshelwood model. The adsorption constant calculated from the linear form of this model, K-ads (7.79 x 10(-2)) was found quietly higher than Kad, deducted from isotherm adsorption. At high initial concentrations of MeO, the low efficiency of the photocatalytic process may result from the inhibition of the by-products during the dye transformation. This inhibition was estimated by comparing to observed and estimated half-lives time.