It was demonstrated in shake-ask experiments that the sulphur-oxidizing microbiota of municipal sludges can be used at 53 degrees C for heavy-metal leaching. Five sludges were tested and the average final pH, oxidation/reduction potential and SO42- concentration after 30 days were 2.8, 237 mV and 5668 mg/l respectively. Ferric chloride was added to enhance the redox potential and to lower pH, which resulted in average values of 409 mV and 1.86 respectively. The average solubilisation of Cd, Cu, Cr, Mn, Ni, Pb, Zn, K and P after ferric chloride addition was 35.6 +/- 20.6%, 65.9 +/- 15.1%, 28.5 +/- 13.5%, 74.0 +/- 10.0%, 60.3 +/- 13.1%, 33.7 +/- 27.6%, 83.9 +/- 6.2%, 39.0 +/- 16.6 and 18.2 +/- 15.8% respectively. The present process enhanced the sludge dewaterability compared to the conventional thermophilic digestion. During the leaching batch process, the volatile and volatile suspended solids were degraded to the same level as observed when the conventional thermophilic digestion was used as control.