Ab initio self-consistent-field (SCF), second-order Moller-Plesset (MP2) and coupled cluster theory including single and double excitations (CCSD) calculations have been used to examine the structure and to help in the assignment of the vibrational absorption of the primary insertion product in the boron-methane reaction. Unlike CH3AlH, CH3BH has no symmetry; C-s structures are first-order saddle points, the imaginary frequency being the torsional mode around the B-C bond. The insertion reaction is exothermic by about 50 kcal/mol. The energetics of several CH3BH dissociation or rearrangement channels have been examined. Dissociation into CH2BH + H or rearrangement to CH2BH2 is energetically favorable. This agree with recent matrix isolation study : CH2BH and CH2BH2 were the major products after reaction of boron with methane. The title species can be identified experimentally by its strongest calculated IR band at 2680 cm(-1) (CCSD/DZP) corresponding to the B-H stretching mode.