The total content of six heavy metals (Cu, Zn, Pb, Cr, Ni and Cd), the Cd/Zn ratio and the zinc equivalent were determined for composts produced from municipal solid waste (MSW) both with and without the removal of inert components prior to the beginning of composting. For two of the composts, one produced by summer and the other by winter composting, the content of heavy metals and the degree of humification were monitored at regular intervals throughout the composting period, and the correlation between the solubility/extractability of the heavy metals with various extractants (1-120, KN03, EDTA and DTPA) and the degree of humification was calculated. The total content of heavy metals increased appreciably during composting due in part to mineralization of the organic matter but also to the 'purification' of the compost (removal of inert materials such as plastic and glass) during the composting period. The solubility/ extractability of the various heavy metals in the summer and winter composts showed similar trends. High correlations between heavy metal content and degree of humification were found for all of the elements, with most of the extractants, for the winter compost, whereas for the summer compost this correlation was high only for Zn.