Abstract: The thermodynamic equilibrium calculations were performed to reveal the distributions of the trace elements (arsenic and selenium) during combustion of six Chinese coals. Actual coal compositions and combustion conditions were utilized in all simulations. The interaction between trace element and halogen and that between trace element and major minerals were considered during calculation. The rank and source of coals have significant effect on the distributions of arsenic. Arsenic mobility has varied dramatically when AsO(g) is ignored from the database. The constrained simulation results appear to more actually reflect the distribution of arsenic during coal combustion. The predicted modes of occurrence of selenium for the six coals are nearly identical.