The characterization of waters produced during in situ combustion of a tar sand deposit near Vernal, Utah, is presented. The water samples were collected during two different field experiments. Analysis of the inorganic constituents by standard methods indicated that ammonium, sulfate, and chloride were the predominant ions. Fractions of the organic material, defined as acid and base extracts, were obtained by liquid-liquid extraction using ethyl ether. Gravimetrically, the acid extracts comprised more than 70 percent of the extractable organic material. Identification of the components in the acid extracts was accomplished by using combined gas chromatography-mass spectroscopy (GC-MS) after methylation with diazomethane. The base extracts were found to be more complex and could not be studied directly with GC-MS. Of the major organic compounds identified, carboxylic acids, particularly acetic acid, were found to be the most abundant. Phenols, lactones, and pyridines were also identified.