The structures and thermodynamic properties of methyl derivatives of ammonia-borane (BH3NH3, AB) have been studied with the frameworks of density functional theory and second-order Moller-Plesset perturbation theory. It is found that, with respect to pure AB, methyl ammonia-boranes show higher complexation energies and lower reaction enthalpies for the release of H-2, together with a slight increment of the activation barrier. These results indicate that the methyl substitution can enhance the reversibility of the system and prevent the formation of BH3/NH3, but no enhancement of the release rate of H-2 can be expected.