Ad situ remediation techniques appear as an interesting alternative for the treatment of polluted solid matrices that are not amenable to in situ remediation, such as soils characterized by high contents of clay and organic matter coupled to low hydraulic conductivity and permeability. In this work, the removal of 2,4-dichlorophenoxyacetic acid (2,4-D) from an agricultural mineral soil with high contents of clay and organic matter was evaluated in lab-scale slurry bioreactors under both aerobic and anaerobic (sulphate-reducing) conditions. Also, the effect of an additional carbon source (sucrose) on 2,4-D removal was assessed. A soil 48% clay and 4% organic matter was sterilized and contaminated with 300 mg 2,4-D/kg dry matrix and subsequently treated in both aerobic (A-SB) and sulphate-reducing slurry bioreactors (SR-SB), with and without 1 g/L sucrose. Both SBs received a seed (20%, v/v) acclimated to 2,4-D from aerobic and sulphate-reducing continuous, complete mix reactors. In this study, aerobic conditions were more favourable for the degradation of 2,4 D in terms of the overall removal efficiency (93%), compared to 25% under sulphate-reducing conditions at 14 days treatment (p < 0.0001). However, the specific removal rate of SR-SB (in mg of 2,4-D removed per mg VSS biomass per day) was significantly higher (ranging between 10 and 13 mg 2,4-D/(mg VSS seed day)) than that in A-SB (2.7 mg 2,4-D/(mg VSS seed day)) (p < 0.0002). This difference was ascribed to the fact that the aerobic seed was much denser than the sulphate-reducing inoculum. The effect of the sucrose amendment on 2,4-D overall removal depended on the electron acceptor used: aerobic removal was not affected by the sucrose supplementation, whereas the removal in the SR-SB receiving sucrose was higher (p < 0.001) than in units without sucrose. However, the SR-SB without sucrose still showed an important 2,4-D removal (up to 18%) suggesting that 2,4-D could be used as substrate. The 2,4-D-clastic bacteria were present in all reactors during the incubation: aerobic 2,4-D-clastic bacteria ranged between 6 and 8 logs, whereas the sulphate-reducing 2,4-D-clastic population ranged between 5 and 7 logs. Overall, the slurry bioreactor ad situ bioremediation technique seems to be an effective alternative for the treatment of mineral agricultural soils characterized by a fine texture and high contents of organic matter, polluted with herbicide 2,4-D. (c) 2006 Published by Elsevier Ltd.