A thermodynamic analysis has determined in detail the speciation of Hg, Se, and As, which are the most volatile trace elements in the flue gas from burning bituminous coal, depending on the temperature (400-1800 K), the atmosphere (reducing or oxidizing conditions), and the chlorine content. The amount of chlorine in the system greatly affects the identity of stable species; the relative affinities of the three elements to chlorine are in the order: Se > As > Hg under oxidizing conditions at both high (1100 K) and low (600 K) temperatures. Interactions between the three elements are investigated; AsSe(g) and HgSe(g) are found to be possibly present as the major species in the local CO-enriched zone in a boiler. The competing reactions of these three elements for oxygen, hydrogen, and chlorine are also discussed,