Municipal solid waste incineration (MSWI) significantly reduces volume and mass by as much as 80%, prolonging the life span of landfills. The concentration of heavy metals in the ash and their ability to leach into ground water is a serious concern when siting and designing MSWI ash landfills. Improved technology captures most heavy metals in the ash. The distribution of elements among the different ash particle sizes was determined by NAA. The bottom ash residue was separated into fractions ranging from 9.5 mm to 0.3 mm. The fly ash was separated into fractions from 250 m to 20 m. Landfills usually bury a mixture of both. The combined ash was separated into fractions over the entire range from >9.5 mm to <20 m. Thermal and epithermal neutron irradiations of size fractionated MSW bottom, fly and combined ash were performed to determine the distribution of various metals within the ash. Compared to normal soil, the ashes contained elevated amounts of numerous elements. Concentrations of the more enriched elements (Ag, Cd, Cr, Cu, Hg, Sb, Se, Sr and Zn) in fly ash were of particular interest as source markers.