To further understand the speciation and partitioning of mercury species in combustion systems, it is necessary to be able to identify and quantitate the various forms of oxidized mercury. Currently accepted methods for speciating mercury (Ontario Hydro Method, EPA Method 29, MESA Method) only separate elemental and oxidized forms of mercury at best, and they are expensive. This paper describes research on the application of cryogenics to the sampling and separation of oxidized forms of mercury from combustion flue gas. This research has resulted in the development and demonstration of a prototype cryogenic sampling device for the determination of oxidized mercury in flue gas. Preliminary results are encouraging, as they show that mercuric chloride and mercuric oxide can be effectively separated through the prototype device. Further development, refinement, and application of this device are expected to generate data that are critically needed to develop effective control technologies for mercury. (C) 2000