The metals speciation in all sludge samples from the different Wastewater Treatment Plants (WWTPs) indicated that, Mn, Ni and Zn were most abundant in the exchangeable, carbonate and Fe/Mn-oxide forms which are the easily assimilable fractions and showed the greatest degree of their mobility, while Cd, Cu, Cr and Fe were major in the organic and sulfide (exhibit some degree of mobility), and the residual form (inert phase) which, corresponds to the part of the metals which cannot be mobilized. Cement kiln dust significantly reduced the availability of metals by chemical modification of their chemical speciation into less available forms. (c) 2008 Elsevier B.V. All rights reserved.