The release of toxic trace elements from the combustion of fuels in power plants is an environmental issue of increasing concern. Trace element release mechanisms are known to be complex and may depend on the levels of chlorine and sulfur in the fuel burned. This dependency has been investigated experimentally by injecting first hydrogen chlorine, and then sulfur dioxide, into a suspension-firing reactor burning wood-bark under simulated fluidized bed combustion conditions. Data interpretation has been aided by parallel theoretical studies, using a thermodynamic equilibrium model based on the principle of Gibbs free energy minimization. The influence of chlorine and sulfur was confirmed for some, but not all, of the 10 trace elements studied. Injection of HCl served to increase the emission of Cd, but to reduce emission of Cu, Mn, Zn, and possibly Cr and Ni. Injection Of SO2 served to increase emissions of Cd, but reduced emissions of As and Hg. The thermodynamic model proved valuable for interpreting the experimental data. However, evidence suggests that predictions not validated by measurement should be treated with caution, given the complexity of the systems being studied.