The concentrations and isotopic compositions of mercury (Hg) in surface sediments of the Pearl River Estuary (PRE) and the South China Sea (SCS) were analyzed. The data revealed significant differences between the total Hg (THg) in fine-grained sediments collected from the PRE (8-251 mu g kg(-1)) and those collected from the SCS (12-83 mu g kg(-1)). Large spatial variations in Hg isotopic compositions were observed in the SCS (delta Hg-202, from -2.82 to -2.10 parts per thousand; Delta Hg-199, from +0.21 to +0.45 parts per thousand) and PRE (delta Hg-202, from -2.80 to -0.68 parts per thousand; Delta Hg-199, from -0.15 to +0.16 parts per thousand). The large positive Delta Hg-199 in the SCS indicated that a fraction of Hg has undergone Hg2+ photoreduction processes prior to incorporation into the sediments. The relatively negative Delta Hg-199 values in the PRE indicated that photoreduction of Hg is not the primary route for the removal of Hg from the water column. The riverine input of fine particles played an important role in transporting Hg to the PRE sediments. In the deep ocean bed of the SCS, source-related signatures of Hg isotopes may have been altered by natural geochemical processes (e.g., Hg2+ photoreduction and preferential adsorption processes). Using Hg isotope compositions, we estimate that river deliveries of Hg from industrial and urban sources and natural soils could be the main inputs of Hg to the PRE. However, the use of Hg isotopes as tracers in source attribution could be limited because of the isotope fractionation by natural processes in the SCS.