Mercury emission from coal-fired power plants is causing great concern. Thermodynamic equilibrium calculations are commonly used for prediction of Hg speciation in the flue-gas. According to representative calculations, at temperatures below 725 K all Hg should exist in oxidized forms. However, Hg speciation measurements for flue-gas at much lower temperature showed a wide range of oxidized Hg fraction from 30 to 95%. A hypothesis that Hg equilibrium is frozen as the flue-gas cools could describe the data. However, a different interpretation of the measured results can be given based on kinetic considerations. Here, the oxidized Hg fraction is limited by the reaction rate of chlorine with elemental Hg in the flue-gas, while the differences in the degree of oxidation can largely be attributed to different time-temperature histories for the flue-gases. The residence time of flue-gas is likely to be a governing factor for the Hg speciation. Crown Copyright (C) 2002 Published by Elsevier Science Ltd. All rights reserved.