Immobilization of heavy metals contained in fly ash was conducted by mixing fly ash with asphalt, sulfur and sodium hydroxide in a bi-axial kneader heated at 403 K. From XRD analysis of the products after treatment of a mixture of CuCl, CuCl(2)center dot 2H(2)O 20, CuO, CdCl(2)center dot 2.5H(2)O, HgCl2 and HgO with sulfur and sodium hydroxide, it was confirmed that all heavy metal compounds were sulfurated to form metal sulfides at 403 K. On the other hand, zinc compounds (ZnCl2 and ZnO) were not converted to zinc sulfide, but were converted into water insoluble compounds. In this treatment, chromium(IV) oxide (CrO3) was reduced to chromium(III) oxide (Cr2O3). The amount of sulfur and sodium hydroxide necessary for complete immobilization of the heavy metals was estimated by the total metal content in fly ash, on the assumption that all heavy metals reacted with sulfur and sodium hydroxide in accordance with the following relations: 2MCl(2) + 3S + 6NaOH -> 2MS + Na2SO3 + 4NaCl + 3H(2)O 2MO + 3S + 2NaOH -> 2MS + Na2SO3 + H2O.