The presence of heavy metals in incinerator flue gases is of environmental concern due to their associated toxic properties. The heavy metals are associated with flyash. Several flyash samples from a range of municipal solid waste incinerators have been analysed by ICP and coupled SEM-EDX to determine their heavy metal concentration and also heavy metal composition in relation to physical morphology. Flyash is shown to contain a wide variety of heavy metals and consists of complex potassium, sodium and calcium aluminosilicates which form spherical particles with an associated amorphous and crystalline phase with an enriched metal content. An experimental reactor is described which investigates the interaction of heavy metal species with flyash under simulated furnace/flue gas conditions. The literature is reviewed in relation to the vaporization and condensation reactions of heavy metals under waste incinerator combustion and flue gas conditions.