The speciation of heavy metals can play a major role in the toxicity of those metals in sewage sludge following land application. Tessier sequential extraction was used to investigate the changes in Ni and Cr speciation in sewage sludge treated by forced-aeration composting. Ni and Cr concentrations increased 30.4% and 36.0%, respectively, during the composting process, with H2O and CO2 volatilization being a major contributor to the change. It was found that the exchangeable, carbonate-bound, Fe-Mn oxide-bound, and organic matter-bound Ni and Cr were transformed to residual fractions. For Cr, the carbonate-bound, Fe-Mn oxide-bound, and organic matter-bound fractions were major contributors, while organic matter-bound Ni was a major contributor to the residual fraction. Composting appeared to reduce Ni and Cr availability by stabilizing the two metals and making them more stable and less mobile. (c) 2006 Elsevier B.V. All rights reserved.