Mercury-thiourea complex ion chromatography is the core of a relatively new approach to Hg speciation analysis that is sensitive enough to accurately quantitate monomethyl Hg at ultratrace environmental levels. In this chapter, a detailed description of an updated system chemistry and operating conditions for performing mercury speciation analysis using inductively coupled plasma mass spectrometry are presented. The new operating conditions are very stable, highly-sensitive, and free of problems that afflicted earlier versions of the system. The potential for obtaining accurate results using the system is demonstrated by the excellent performance metrics obtained and the close agreement of these results with consensus values of monomethyl Hg in water and sediment reference materials and inter-lab comparison samples. The ability to quantitate mercuric and monoethyl along with monomethyl forms of Hg is a feature of this approach that likely will prove useful in future studies of environmental systems.