The identities and properties of mercury species in crude oil were investigated. Dimethylmercury (DMHg) and elemental mercury (Hg-0) were independently quantified as volatile species evaporated from a single crude oil using selective trapping. Henry's law constants of Hg-0 and DMHg in toluene were determined by measuring concentrations in vapor/liquid equilibrium over a range of temperatures. Of the two, elemental mercury, as a dissolved species in the two-phase toluene liquid/vapor systems, has the higher fugacity and Henry's constant. Nonvolatile forms of mercury associated with asphaltenes were identified using differential solubility methods. Asphaltene mercury was found to be a significant fraction of the total mercury concentration in the crude oils that were examined. The newly developed analytical protocols have assisted in understanding how mercury and its compounds distribute in petroleum processing.