Quantum chemical methods were used to investigate the OH initiated atmospheric degradation of methanimine, CH2=NH, the major primary product in the atmospheric photo-oxidation of methylamine, CH3NH2. Energies of stationary points on potential energy surfaces of reaction were calculated using multireference perturbation theory and coupled cluster theory. The results show that hydrogen abstraction dominates over the addition route in the CH2=NH + OH reaction, and that the major primary product is HCN, while HNC and CHONH2 are minor primary products. HNC is found to react with OH exclusively via addition to the carbon atom followed by O-H scission leading to HNCO; N2O is not a product in the atmospheric photo-oxidation of HNC. Additional G4 calculations of the CH2=NH + O-3 reaction show that this is too slow to be of importance at atmospheric conditions. Rate coefficients for the CH2=NH + OH and HNC + OH reactions were calculated as a function of temperature and pressure using a master equation model based on the coupled cluster theory results. The rate coefficients for OH reaction with CH2=NH and HNC at 1000 mbar and room temperature are calculated to be 3.0 x 10(-12) and 1.3 x 10(-11) cm(3) molecule(-1) s(-1), respectively. The atmospheric fate of CH2=NH is discussed and a gas phase photo-oxidation mechanism is presented.