The reactions of a series of hydrocarbon- and oxygen-containing organic compounds have-been studied in supercritical water, 15% aqueous formic acid, and 15% aqueous sodium formate at 460 degrees C. For each substrate, a solution in cyclohexane at 460 degrees C was also examined in order to distinguish thermolytic (mainly radical) reactions from the ionic reactions in the aqueous systems. Most substrates showed significant change within 1 h. Biphenyl, 1,1’-binaphthyl, naphthalene, and phenanthrene were essentially unreactive. 1-Benzylnaphthalene underwent only slow thermolysis, whereas 1-benzyltetralin underwent rapid homolysis in all media. Cyclohexylbenzene and 1-cyclohexyl-naphthalene showed similar trends in reaction in all media, as did hexylbenzene and 1-decylnaphthalene. The conversion rates for the n-alkyl-substituted aromatics were noticeably higher than those for the cyclohexyl-substituted aromatics. Dibenzofuran was unreactive, and diphenyl ether only underwent a slow base-catalyzed hydrolysis. Cyclohexyl phenyl ether was highly reactive at short reaction times, and 1-naphthyl phenyl ether was noticeably less reactive. 1-Naphthol and 1-tetralone showed significant conversion in the basic media and there was evidence of reduction in the formate solutions. 1-Octanol showed little reactivity after the shorter reaction time.