The thermolysis of C60H2 to yield C-60 and H-2 was studied by hybrid density functional theory (B3LYP/6-311G**// B3LYP/3-21G). The concerted loss of dihydrogen requires an activation energy of 92 kcal mol(-1) at T = 452 K. An alternative radical mechanism, which is first order in the C60H2, concentration, has an activation energy at 452 K of only 61 kcal mol(-1). Monitoring of the C60H2 decomposition in 1,2-dichloro-[D-4]-benzene solution by NMR spectroscopy indicates a pseudo first-order reaction with an activation energy of 61.38 +/- 2.35 kcal mol(-1). (C) 2002 Elsevier Science B.V. All rights reserved.