Sorbent effects in the microbial uptake of diesel oil were determined for black cotton soil (BCS) and two oil spill clean-up sorbents, ie peat sorb and spill sorb. Biodegradation studies were conducted in mass transfer limited batch slurry microcosms using microorganisms capable of direct interfacial uptake of diesel oil. Under identical loading conditions, the amounts of diesel oil initially loaded on the various sorbents were 178, 288 and 649 mg g(-1) for BCS, spill sorb and peat sorb, respectively. Total biodegradation of sorbed diesel was comparable for all the sorbents (45-52 mg), however, the biodegradation rates were significantly different. Peat sorb demonstrated a distinct initial lag phase, the biodegradation rate in spill sorb was initially slower, whereas biodegradation at a high rate commenced immediately for BCS. The maximum biodegradation rates observed for BCS, spill sorb and peat sorb microcosms were 7.9, 5, and 2.9 mg day(-1), respectively. Thus, the maximum biodegradation rate increased as the diesel oil loading decreased. Our results indicate that spill clean-up sorbents have greater bioavailability limitations compared with soils and this is linked with their significantly higher loading capacity and internal porosity. (c) 2005 Society of Chemical Industry