The photocatalytic degradation of three model pollutants (phenol, 4-chlorophenol, and herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) was performed at room temperature in aqueous suspended mixtures of TiO2 and of two different activated carbons (AC) of commercial origin. The kinetics of disappearance of the pollutants followed an apparent first-order rate, whose constant was selected as the best parameter (independent of pollutant concentrations) to determine the influence of each AC on the photoactivity of titania. Remarkable different effects were observed in the kinetics of disappearance of the pollutants as well as in the kinetics of appearance and disappearance of the intermediate products detected. Each pollutant was more rapidly photodegraded in the mixed system which contained H-type activated carbon. In the case of phenol, a maximum synergy factor R equal to 2.5 was detected when the mass ratio (TiO2/AC(H-type)) corresponded to (50/10). An identical synergy factor (2.4) was found for 4-chlorophenol degradation, whereas a smaller one (R = 1.3) was found for 2,4-D because of its poor solubility. On the contrary, the L-type activated carbon inhibited titania's photoactivity with R < 1. In addition, significant differences in intermediate product distributions were observed as a function of the type of AC. The synergy or inhibition effects and the formation of the different intermediate products detected have been correlated to the origins and the properties of the two AC employed. When performing the "helio-photocatalytic" degradation of 4-chlorophenol with an extrapolation volume factor of 12,500 in a large-scale solar pilot plant, an identical synergy factor of 2.4 was found, thus confirming the transpositivity of laboratory experiments to large solar setups. The synergy effect was not destroyed when reusing the double-phase photocatalyst. This photocatalytic system, associating titania with H-type activated carbon, may appear as a new performing one, more efficient with a shorter time necessary for decontaminating diluted used waters. It could be of interest in producing detoxified and/or drinking water in dry sunny areas.