To facilitate the understanding of the fate of metals during incineration the speciation of metals is critical. Thus, an experimental study to determine the speciation of metals during incineration was performed. Aqueous metal salts of arsenic, cadmium, chromium, and lead were injected into a laboratory reactor, post-flame. Aerosols were captured on a glass fiber filter by isokinetic sampling. Reactor temperature was varied from 600 to 1100°C and the stoichiometric ratio was varied from 0.95 to 1.25. The Reference Intensity Method (RIM) of quantitative X-ray diffraction analysis combined with X-ray transmission was utilized to determine the speciation of aerosols removed from the combustion environment.