CRUDE OILS are a highly complex combination of hydrocarbons; heterocyclic compounds of nitrogen, oxygen, and sulfur; organometallic compounds; inorganic sediment; and water. Approximately 600 different hydrocarbons have been identified in crude oil, and it is likely that thousands of compounds occur, many of which probably will never be identified. In a study sponsored by the American Petroleum Institute (API), nearly 300 individual hydrocarbons were identified in Ponca City, Oklahoma, crude oil [1,2]. Some 200 individual sulfur compounds were identified in a 20-year systematic study of four crude oils [3]. Not only is the composition of crude oil highly complex, it is also highly variable from field-to-field, and even within a given field it is likely to exhibit inhomogeneity. Physical and chemical characterization of this complex mixture is further complicated for the analyst by the fact that crude oils are not pure solutions, but commonly contain colloidally-suspended components, dispersed solids, and emulsified water.