The modes of occurrence of mercury in coals from Guizhou, P.R. China were investigated using sequential extraction procedures and density separation experiments. The sequential extraction tests were carried out using 32 coals sampled from the Longtan Group of the Permian system in Guizhou Province. Eight coal samples collected from the same area were divided into 10 density fractions between < 1.4 and > 2.8 g/cm(3) by separation in a heavy-liquid mixture. Combining the results of mineral transformation and of the amount of mercury leached during the leaching experiments with the results of the correlation between mercury and mineral phases and organic matter from density separation experiments, the authors concluded that mercury in the coals examined mainly exists in pyrite. The studies also showed that most of mercury in these coals can be brought into the environment easily by acid mine drainage. It is inferred that a rather large part of Hg in coals from Guizhou may be removed by using conventional coal cleaning techniques.