A novel system for mercury speciation analysis using high-pressure ion chromatography (IC) has been developed and validated. Its chemistry permits separation of the two most abundant forms of Hg in natural waters, soils, sediments, and biota-monomethyl Hg (CH3Hg+) and mercuric Hg (Hg2+)-on the basis of the difference in charge of their respective thiourea (S=C(NH2)20) complexes. Once separated, both species are converted to Hg0 on-line and quantified by cold-vapor atomic fluorescence spectrometry (CVAFS). A column containing thiol-functionalized silica resin installed in the sample loop of the IC system traps Hg2+ and CH3Hg+ from prepared sample solutions without retaining interfering sample matrix components. The resulting matrix-independent chemistry permits external calibration of the system and a high sample throughput ( approximately 6 samples per hour). The system's accuracy has been validated with environmentally relevant reference materials. Figures of merit for the system, an average precision of approximately 2.5% and an absolute detection limit of <1 pg, are comparable to state-of-the-art gas chromatography approaches for Hg speciation analysis (ethylation/GC-CVAFS) and meet or exceed those of all extant LC systems. Low relative detection limits are attainable through preconcentration onto the thiol resin.