The photodegradation processes using semi-conductor like TiO2 as a catalyst, provide new methods for waste water treatment. In this method the step of adsorption of organic compounds on the semi-conductor surface could have a prominent influence. We describe the adsorption phenomen for two classes of organic molecules (phenolic compounds and dicarboxylic acids) studied by diffuse reflectance infrared spectroscopy (IRFT-DRIFT mode). The relations between adsorption and degradation mechanisms has been approached. In particular, we presente the results of the dicarboxylic acids and phenolic compounds adsorption on TiO2 (P25) in aqueous phase. The Langmuir model gives a good representation of the adsorption of the boths studies organic compound families. In the case of the diacid, the DRIFT spectroscopy of loaded TiO2 enables to demonstrate the formation of a dicarboxylate film on the TiO2. The adsorption of phenol molecules with the plane of the aromatic ring parallel to the TiO2 surface is observed and confirmed by skeletal pattern simplification and the disappearance of the bending O-H absorption vibration shown phenolate ions formation and bonding with TiO2.