Semi-empirical PM3 molecular orbital calculations have been used to investigate the reaction pathways of pyrolysis of pyridine. All the transition states and intermediates of the reaction path were optimized. The probable pathways are estimated from the activation energies calculated and compared with known experimental results. The heats of formation of pyridine radicals are calculated to be 76.1, 80.6, and 79.5 kcal/mol for 2- (2A), 3- (3A) and l-pyridyl radicals (3A), respectively. The weakest bonds on the radicals were assumed to be broken and gave thr decomposition fragments successively. (C) 2000 Elsevier Science Ltd. All rights reserved.