In this study, a comparison of the mobility of soil mercury with two different extractable treatments (water-treated and (NH4)(2)S2O3-treated) was carried out in soil samples collected from Wanshan Mercury Mine (WSMM), Guizhou, SW China. Substantially higher levels of mobilized Hg were found in (NH4)(2)S2O3-extracted (1.22 to 2.41 mu g g(-1)) compared to the water-extracted soil samples (0.05 to 0.49 mu g g(-1)). To understand the geochemical behavior of Hg during Hg mobilization, and to identify the potential hazard of Hg in soil, Hg isotope compositions of total Hg, water-soluble Hg and (NH4)(2)S2O3-extractable Hg in WSMM soil were measured by using multiple collectors coupled plasma mass spectrometer (MC-ICP-MS). A large variation of mass-dependent fractionation (MDF) of Hg was observed (delta Hg-202 of -0.29-1.59 parts per thousand) between the extractable Hg species and the total Hg in soil. Mass independent fractionation (MIF) in Delta Hg-199 ranged from -0.07 to 0.07 parts per thousand, which were statistically insignificant. The experimental data (delta Hg-202 values) revealed that water-soluble (delta Hg-202=0.70 +/- 0.13 parts per thousand, n=8) and (NH4)(2)S2O3-extractable (delta Hg-202=1.28 +/- 0.25 parts per thousand, n=8) Hg species were enriched in heavier Hg isotopes by 0.72 parts per thousand and 1.30 parts per thousand relative to total Hg in soil samples, respectively. The results suggest that the bioavailable fraction of Hg in soil possesses heavier Hg isotope values than total Hg in soil. To understand mercury isotope fractionation in the biogeochemical cycling processes in soil, it is of importance to measure Hg isotope compositions of different Hg species. (C) 2012 Elsevier B.V. All rights reserved.