The biological oxidation of elemental sulphur (S-o) added to three alkaline composts prepared with a range of organic wastes (CC, melon crop residues; MC, mixed manures; and BC, pine bark) to reduce their pH was studied. The titration curves showed that to achieve an equivalent pH drop, compost CC needed a larger dose of S-o than did composts MC and BC. The acidification efficiency was high in the three composts (>= 53%), but the pH reduction obtained from the titration curves in MC and BC composts was lower than expected. S-o oxidation in amended composts was found to be related to pH and CaCO3 content decreases, and to the rise in CaSO4 and electrical conductivity levels. A remarkable increase in the autotrophic bacteria population and a slight increase in heterotrophic bacteria along with S-o oxidation were recorded. Actinomycetes, fungi and yeasts were not affected by the addition of S-o to composts. (C) 2006 Elsevier Ltd. All rights reserved.