Ab initio quantum chemical methods are applied to the study of ethylene decomposition to acetylene and molecular hydrogen in the ground electronic state. Results are reported on three different pathways for ethylene decomposition-two stepwise processes involving a hydrogen transfer followed by 1,1 elimination of H-2, or vice versa, and a 1,2 elimination. The latter proceeds through an energy maximum with two imaginary frequencies, rather than one as for conventional transition states. Ethylidene and vinylidene are predicted to be stationary points on the C2H4 and C2H2 potential energy surfaces, respectively. Recent photochemical studies have observed rotationally hot H-2. It is shown that due to the excess energy available in the photochemical experiments, all three mechanisms can give rise to rotationally hot H-2 when proper account is taken of the transverse vibrational modes along the reaction paths.