Two sewage sludges, one anaerobically digested, one undigested, were composted with straw using a computer-controlled laboratory composter. Cd, Cr, Cu, Ni, Pb and Zn concentrations in the sludges and composts were analysed by (1) digestion, to assess changes in the total metal content, (2) DTPA (diethylene-triamine-pentaacetic acid) (0.005M) and CaCl₂ (0.5M) extractions, to indicate, respectively, plant-available and exchangeable metals, and (3) sequential extraction, to gain further information about movement between metal fractions during composting and maturation. To achieve composting the anaerobically digested sludge required the addition of vegetable margarine as a high-energy supplement. The undigested sludge/straw mixture reached thermophilic temperatures without the need for a high-energy supplement because of the greater degradable carbon content of this sludge. The sludge total metal content was decreased by dilution with straw because this has a relatively low total metal content. The metal content of the matured compost depends on the balance between dilution by straw, concentration during composting and loss of metal by leaching [Wagner et al, 1990]. There was no significant increase in total concentrations of Cu, Pb and Zn in the final compost compared to the original sludges, but it was found that Cd and Ni concentrations could increase relative to the other metals. There is a positive correlation between organically-bound Cu (and other metals) and the increase in humification during composting, whereas oxidation decreases the proportion of metals associated with the sulphide fraction. Both plant-available and exchangeable metals tend to increase during the active phase and decrease during maturation. It is suggested that active aeration leads to increases in metal mobility and this is followed by binding in more stable forms as humification increases during maturation. Therefore, while the total heavy metal content would normally increase during composting, the process can also lead to a decrease in environmentally-available forms of heavy metals.