Biphenyl, or more accurately biaryl, linkages represent the most refractory cross-links in fossil fuel resources, They are essentially unreactive thermally and in supercritical water at 460 degrees C. Many biaryls do, however, undergo bond cleavage and heteroatom removal under reducing conditions in supercritical water. 2-Arylpyridines, -quinolines, and -indoles are most reactive in 15% aqueous formic acid whereas the corresponding 2-arylthiophene and -benzothiophene derivatives are more reactive in 15% aqueous sodium formate. Major hydrocarbon products are benzene and naphthalene and their C-1-C-4 alkylated derivatives. In most cases where conversion is not quantitative after 1 h at 460 degrees C, the product slate consists of hydrogenated and cleaved products which, given more reaction time, are on the pathway toward complete heteroatom removal.