The influence of composting on sewage sludges contaminated by Cd or by high concentrations of Cd, Cu, Ni and Zn was studied. Independently of the contamination which existed the composting process led to a loss of weight, as shown by the decrease in organic matter however metal contamination did influence microbial populations and led to lower mineralization and humification being achieved. Contamination by Cd led to a much lower degree of nitrification during composting. Hydrolytic enzyme activities showed that large quantities of heavy metals had a negative effect on urease synthesis. Similarly, contamination negatively affected phosphatase; although not beta-glucosidase, which increased with the level of contamination. The composting process rendered insoluble that portion of the metals which could be extracted with DTPA. CaCl2 or H2O by forming metal-humus complexes. Of the metals which were present at high quantity in the sewage sludges (Ni, Cu, Zn and Cd), Ni was the most easily extracted by H2O or CaCl2 and was therefore the most weakly bound to the organic matrix Zn, in contrast, remained strongly bound.