To better monitor and control Hg emissions from coal-fired power plants, quantification of Hg contents in coal is essential. Thermal decomposition is widely used to pretreat solid samples. The complete thermal release of Hg in coal samples is a premise for obtaining accurate results of measured Hg contents. The recommended heating temperature is 800 degrees C for coal samples. The research work in this paper has verified the existence of an abnormal phenomenon, where some Hg occurrences in typical coal at 800 degrees C could not fully decompose. The temperature programmed pyrolysis was designed to validate the assumption and determine the optimum decomposition temperature. The results demonstrate that Hg combined with montmorillonite mineral fully decomposes at temperatures above 800 degrees C. The obtained results help to clarify the relationship between the decomposition temperature and Hg occurrence. The study offers good guidance to improve the accuracy of measured Hg contents by thermal decomposition, thus expanding the laboratory and industrial application. (C) 2016 Elsevier B.V. All rights reserved.