Our study investigated the adsorption/desorption by/from activated sludge flocs, dispersed in river water or in diluted wastewater, of organic compounds (C 11 -LAS, azoalbumin and azocasein) at concentrations relevant to environmental conditions. Activated sludge flocs, used as a model of biological aggregates, are characterized by a very heterogeneous matrix able to sorb the three organic compounds tested at 4°C. The adsorbed amount of C 11 -LAS by activated sludge flocs was higher than that of azocasein or azoalbumin, as shown by the Freundlich parameters (K ads =8.6±1.7, 1.6±0.3 and 0.3±0.1μmol 1−1/n g −1 l 1/n for C 11 -LAS, azocasein and azoalbumin, respectively; n=3 sludges).C 11 -LAS sorption from activated sludge appeared to be partially reversible in river water, while a marked hysteresis phenomenon was observed for azocasein and azoalbumin, implying a low degree of reversibility in their exchange between activated sludge and river water. It has also been displayed that the conductivity variation of bulk water (comprised between 214 and 838μScm −1 ) exerted no dramatic effect on the C 11 -LAS desorption from activated sludge flocs, while a little effect of it on azocasein desorption was observed.Thus, biological aggregates as activated sludge flocs can serve as an intermediate carrier for C 11 -LAS, while it represents a sink for proteins.