Ten sulfur-containing compounds were selected to serve as models for the sulfur-containing impurities in heavy oils. They were subjected to aquathermolysis in water, 15% aqueous formic acid, and 15% aqueous sodium formate solution for 7 min at 460 degrees C. Refractory materials were rerun over 1 h at the same temperature. In each instance the sulfur compound was exposed to the same time/temperature regime in cyclohexane in order to compare any thermal decomposition with the aqueous results. Thiophene (5) was completely unchanged, and benzo[b]thiophene (33) showed little (<4%) or no change in any medium over 1 h. Diphenyl sulfide (44) was rapidly reduced to benzene (2) in formic acid over 1 h but showed little change in other media. Likewise, thiophenol (22) was rapidly converted to benzene (2) over 1 h in aqueous formic acid and more slowly in aqueous sodium formate, where diphenyl sulfide was the major product. In water it gave mainly diphenyl sulfide (44). However, even in cyclohexane considerable quantities of benzene (2) and diphenyl sulfide (44) were produced over 1 h. 1-Naphthalenethiol (43) paralleled the reactions of thiophenol (22). Cyclohexyl phenyl sulfide (45) showed high conversions in all four media over 7 min to give mixed product slates. Again, acid-catalyzed reduction to benzene (2) and 1-methylcyclopentene (1) was dominant in the aqueous formic acid. 1-Naphthyl phenyl sulfide (52) could be completely desulfurized in formic acid, giving benzene (2) and naphthalene (32) in 1 h along with minor amounts of secondary products. In water an equilibrium developed between the starting material (52), 1,1’-dinaphthyl sulfide (55) and diphenyl sulfide (44). Cyclohexane gave a comparatively low conversion and aqueous sodium formate a high conversion, but in both cases a long product slate was seen. 1-Phenylthiotetralin (51) was highly reactive in all four media. Desulfurization was complete within 7 min in formic acid to give benzene (2), naphthalene (32), and tetralin (30). Significant extrusion of sulfur was also seen in the runs in water and sodium formate, although thiophenol (22) was still a major product in each case. Dioctyl sulfide (50) showed almost complete conversion after 7 min to give mainly the hydrocarbons 1-heptene (8) and 1-octene (12). Tetrahydrothiophene (10) was found to be reactive in all four solvents after 1 h, yet the products were presumably volatile hydrocarbons and were not identified.