In order to evaluate the environmental impact resulting from co-combustion of oily sludges, a detailed characterisation of the ashes produced was carried out. The main bulk composition and the trace elements were determined, thus allowing a comparison of the ashes produced from coal to those from co-combustion of the residue with and without Limestone. The leachability was evaluated with two different standards, a European Norm and the US TCLP. The results differed according to the leaching pH dependence. Of the trace metals analysed, only Cr failed to meet the limits of natural groundwater by the EN procedures. The utilisation of limestone, thus capturing sulphates and part of Cl, had an adverse effect over the leachability. Cr was especially sensible, as the toxic hexavalent form was favoured in this case. The LOI of the ashes was found to be very high. As several mechanisms are believed to contribute to the overall result, low ashing temperatures are required for ashes from residues containing Cl, alkaline salts and carbonates. TGA was found to be a highly valuable technique to evaluate the unburned material. (C) 2001 Elsevier Science Ltd. All rights reserved.