The OH-initiated photo-oxidation of N -methylmethanimine, CH 3 N=CH 2 , was investigated in the 200 m 3 EUPHORE atmospheric simulation chamber and in a 240 L stainless steel photochemical reactor employing time-resolved online FTIR and high-resolution PTR-ToF-MS instrumentation and in theoretical calculations based on quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations forecast the OH reaction to primarily proceed via H-abstraction from the =CH 2 group and π-system C-addition, whereas H-abstraction from the −CH 3 group is a minor route and forecast that N-addition can be disregarded under atmospheric conditions. Theoretical studies of CH 3 N=CH 2 photolysis and the CH 3 N=CH 2 + O 3 reaction show that these removal processes are too slow to be important in the troposphere. A detailed mechanism for OH-initiated atmospheric degradation of CH 3 N=CH 2 was obtained as part of the theoretical study. The photo-oxidation experiments, obstructed in part by the CH 3 N=CH 2 monomer–trimer equilibrium, surface reactions, and particle formation, find CH 2 =NCHO and CH 3 N=CHOH/CH 2 =NCH 2 OH as the major primary products in a ratio 18:82 ± 3 (3σ-limit). Alignment of the theoretical results to the experimental product distribution results in a rate coefficient, showing a minor pressure dependency under tropospheric conditions and that can be parametrized k ( T ) = 5.70 × 10 –14 × ( T /298 K) 3.18 × exp(1245 K/ T ) cm 3 molecule –1 s –1 with k 298 = 3.7 × 10 –12 cm 3 molecule –1 s –1 . The atmospheric fate of CH 3 N=CH 2 is discussed, and it is concluded that, on a global scale, hydrolysis in the atmospheric aqueous phase to give CH 3 NH 2 + CH 2 O will constitute a dominant loss process. N 2 O will not be formed in the atmospheric gas phase degradation, and there are no indications of nitrosamines and nitramines formed as primary products.