It is known that the acidity and reactivity of the CH bond can be enhanced after ionization. Also, this property plays a pivotal role in proton transfer reaction and in the formation of new molecules. Herein, infrared spectroscopy and high-precision quantum chemical calculations are used to study the neutral and cationic clusters of nitromethane−ammonia (CH 3 NO 2 -NH 3 ). It is found that in the neutral cluster, CH 3 NO 2 and NH 3 are mainly bonded by three intermolecular hydrogen bonds, in which electrostatic contribution plays a major role. After vacuum ultraviolet (VUV) single-photon ionization of CH 3 NO 2 -NH 3 , the positive charge redistributes from the ionized nitrogen atom of NH 3 to the CH 3 NO 2 molecule immediately. Then, the proton of CH 3 NO 2 transfers to NH 3 to form a proton-transferred type structure CH 2 NO 2 -NH 4 + , without any effective energy barrier, due to the positive hyperconjugation of cationic nitromethane. A closed loop of positive charge transfer takes place in the CH 3 NO 2 -NH 3 cluster after VUV ionization. The present work demonstrates that both the proton transfer reaction and charge transfer process have occurred in the ionized CH 3 NO 2 -NH 3 cluster. Moreover, it is found that the proton transfer reaction is a result of the highly acidic CH bond caused by hyperconjugation between the σ (CH) bond and π orbital.