This work investigates, at a laboratory and pilot-scale, the influence of various operating parameters on the combined slurry and solid-phase bioremediation technique for a diesel contaminated soil. For slurry-phase bioreactors (SPB), it has been found that, as far as famine conditions are attained at the end of the react cycle, a low hydraulic retention time and a low slurry recycle ratio allows for a better utilization of the reactor volume. A 7-day slurry-phase bioreactor treatment has been shown to provide enough contaminant removal allowing the soil drawn from the slurry-phase bioreactors to be fed effectively to the solid-phase bioreactors (SoPB) for completing the soil cleanup. However, an important improvement of the solid-phase bioreactor performance has been found using soil additives, namely sand and surfactants. While the first soil additive improves pile porosity and consequently oxygen diffusion, the latter increases contaminant bioavailability. (C) 2003 Elsevier Science B.V. All rights reserved.