Abstract: Ash residues produced from solid waste combustion and certain industrial residues which are sometimes contacted with water for dust control, quenched for temperature reduction or subjected to water separation methods, have lead (Pb) and other heavy metals in sufficient solubility to sometimes fail the USEPA TCLP test and other acid leaching tests. Addition of a water-insoluble precipitating agent, such as pulverized phosphate rock or a precipitating agent coated with a water-insoluble film, during waste generation or collection allows the waste to pass TCLP regulatory testing, reduces Pb leaching under acid conditions and provides a water-insoluble stabilizing seed which will remain with the residue throughout water spraying, quenching or wet processing.