The development of elemental mercury (Hg0) capture technology for coal-fired power plants is essential for achieving the goal of zero emissions from coal derived flue gas. Sorbents such as biomass gasification chars have proved to be effective for mercury capture and offer the added advantage of their low cost since they are sub-products of the thermal conversion process. However, the mercury species captured on the sorbents have not yet been characterized. In this study, a temperature programmed decomposition technique was used to identify the mercury species captured on the sorbents and to clarify the mechanisms responsible for mercury retention. The mercury species formed were observed to be dependent on char characteristics and flue gas composition. The results showed that mercury chloride was the most likely mercury species in a simulated coal combustion atmosphere from the chars obtained from poultry litter, sunflower husks and paper and plastic waste, the last two containing small amounts of mercury sulphate. The mercury compounds identified in the char from the gasification of wood waste were mainly sulphide and sulphate species.