Ammonia is a possible renewable fuel that does not lead to carbon dioxide emission. When blended with hydrocarbons for better engine performance, species such as methylamine (CH3NH2) can be formed. Computational quantum chemistry modeling has been applied to understand the combustion chemistry. Reaction pathways for CH3NH2 and for subsequent chemistry of products have been characterized. First, geometries and vibrational energies of reactants, intermediates, barriers and products were obtained with density functional theory (M06-2X) and the 6-311+G(2df,2p) basis set, followed by single-point evaluations of the energy using the CBS-APNO methodology which approximates a coupled-cluster calculation extrapolated to the infinite basis set limit. Evaluation of known species indicates high accuracy, with deviations of less than 2 kJ mol-1. The results are used to assess the reactivity of CH3NH2 and its product fragments with oxygen, and the thermochemistry of these steps.