Intramolecular methane elimination through a-hydrogen abstraction in M(CH3)(5) (M = Nb, Ta) has been studied in detail with ab initio quantum mechanics calculations. Geometry optimizations were performed with the 3-21G and HW3 (equivalent to the 6-31G*) basis sets. The energies were further evaluated with the MP2/HW3 calculations. Although the M(CH3)(5) reactants significantly favor square-planar structures, the most favorable transition structure for unimolecular methane elimination is close to a trigonal- bipyramidal geometry. Hydrogen abstraction is concerted, although there is significant M-H interaction in the transition structure. The calculated activation free energy is 35.3 and 37.3 kcal/mol for Nb(CH3)(5) and Ta(CH3)(5), respectively, at the best level of calculation. A dimeric mechanism through intermolecular hydrogen abstraction is found to be much lower in activation free energy than the unimolecular mechanism. The stabilization for the dimeric transition structure is mainly due to the formation of an M-CH2-M bridge. Intramolecular methane eliminations in (CH3)(4)M-CH2-M(CH3)(4) were also studied.