This paper describes a laboratory study that examined the effect of thermal treatment at four different temperatures on the behavior of heavy metals in the anaerobically treated primary sludge from the Psittalias wastewater treatment plant. The sewage sludge was found to contain significant amounts of heavy metals (Cr, Cu, Fe, Ni, Pb and Zn). The metal form distribution in the sludge samples which was determining by the application of a sequential extraction procedure revealed that a significant portion of metals was embodied in the organic and reducible fractions. Treatment at 105, 250, 650 and 900 degreesC demonstrated significant conversions of the metals to a less mobile form as well as removal by vaporization. By applying sequential analysis, it was found that most of the metals were removed from the initial mobile phases to more stable ones. Also, significant amounts were transformed to the gaseous phase.