Eleven nitrogen-containing heterocycles were chosen as fossil fuel model compounds : pyridine, quinoline, 2-methylquinoline (quinaldine), isoquinoline, acridine, phenanthridine, 1,2,3,4-tetrahydroquinoline, indole, 2-methylindole, 2,3-dimethylindole, and carbazole. They were heated at 460-degrees-C for 7 min and for 1 h in four different sets of conditions : (i) cyclohexane, (ii) water, (iii) aqueous 15% formic acid, and (iv) aqueous 15% sodium formate. Aquathermolyses under conditions (ii)-(iv) could thus be compared with purely thermal reactions under condition (i). Pyridine is almost unreactive at 460-degrees-C under both thermolysis and aquathermolysis conditions, although in 15% aqueous formic acid at 460-degrees-C for 7 min, small quantities of N-alkylpiperidines were observed; however, on extending the reaction time to 1 h, small amounts of C-alkylated pyridines were formed. Acridine underwent a much higher conversion than phenanthridine, although no nitrogen removal was observed and both compounds showed mainly hydrogenation and/or oxidation products; after 1 h at 460-degrees-C, increased dehydrogenation was observed. Quinoline, 2-methylquinoline, and isoquinoline were much more reactive than the pyridine system, especially in the presence of aqueous 15% formic acid to give mainly hydrogenated derivatives after 7 min; however, after 1 h some dehydrogenation back to the aromatic nuclei was observed. Indole and its methylated derivatives (both mono and di) reacted in a similar way : substantial reduction to indolines was observed in aqueous formic acid after only 7 min; however, after 1 h, the indolines seemingly underwent significant dehydrogenation back to indoles. Carbazole was completely unreactive. Structures of the products from all these reactions have been determined, and reaction sequences for their formation are proposed.