Different studies to unravel the nature of the activity of TiO2 photocatalysts modified with rGO were carried out. The band edge potentials and the band gap energy of the nanocomposites were determined by performing electrochemical impedance and UV-vis diffuse reflectance measurements, respectively. However, no changes were observed when TiO2 is modified with rGO. Nevertheless, the presence of rGO in the hybrid composite led to a low charge transfer resistance across the electrode-electrolyte interface, observing even a tenfold increase in the photocurrent values in methanol photo-oxidation for P25-rGO 1%. Moreover, a higher oxygen reduction current was found when increasing the rGO concentration that could lead to a higher ROS formation. In order to analyze the beneficial properties of the hybrid materials, the influence of rGO doping ratio on oxalic acid photocatalytic degradation and on oxalic acid adsorption onto the nanocomposites surface was studied, both of them under UV-A light irradiation. In addition, the photoactivity of the conduction band electrons and the valence band holes was investigated by performing EPR and transient absorption spectroscopy measurements under UV-A illumination in O-2 or N-2 atmospheres. It was demonstrated that rGO behaved as an electron acceptor. Finally, TAS results under O-2 and N-2 atmospheres proved that the role played by rGO was not as crucial in excess of dissolved O-2 as in N-2 atmosphere. These findings agree with the observed photocatalytic activity and EPR measurements. Nevertheless, generation of HO center dot in N-2 saturated suspensions was highly increased with the addition of rGO.