The transformation behavior of mercury in two Chinese coals (WJP and HYS coal) during sub-critical water treatments was studied in a semi-continuous apparatus. Float-sink method, demineralization and sequential chemical extraction were used to study the occurrence mode of mercury in raw coals and extracted samples. The results show that with increasing temperature, pressure, water flow and extraction time, the removal of mercury increases. During sub-critical water treatment, the content of mercury associated with sulfates, monosulfides, disulfides, organic material, and insoluble forms decreases. The removal efficiency of mercury is almost 100% at 410 degrees C, 15 MPa, 0.58 l/h, and 60 min for HYS coal and 96.7% at 380 degrees C for WJP coal. Under the same temperature and pressure the mercury removal through pyrolysis is less than that through sub-critical water extraction which is an efficient method to remove most mercury from coal. (c) 2005 Elsevier B.V. All rights reserved.