The redox reactions of NH3 and CH3NH2 with N2O4 (NTO) have been studied by ab initio molecular orbital (MO) calculations at the UCCSD(T)∥UB3LYP/6-311+G(3df,2p) level of theory. These reactions are related to the well-known NTO-hydrazine(s) propellant systems. On the basis of the predicted potential energy surfaces, the mechanisms for these reactions were found to be similar to the hydrolysis of NTO and the hypergolic initiation reaction of the NTO-N2H4 mixture, primarily controlled by the conversion of NTO to ONONO2 via very loose transition states (with NH3 and CH3NH2 as spectators in the collision complexes) followed by the rapid attack of ONONO2 at the spectating molecules producing HNO3 and RNO (R = NH2 and CH3NH). The predicted mechanism for the NH3 reaction compares closely with its isoelectronic process NTO + H2O; similarly, the mechanism for the NTO + CH3NH2 reaction also compares closely with its isoelectronic NTO + NH2NH2 reaction. The kinetics for the formation of the final products, HNO3 + RNO (R = NH2, OH, CH3NH, and N2H3), were found to be weakly pressure-dependent at low temperatures and affected by the strengths of H-NH2 and H-OH but not in the RNH2 case. We have also compared the predicted rate constant for the oxidation of NH3 by N2O4 with that for the analogous NH3 + N2O5 recently reported by Sarkar and Bandyopadhyay [J. Phys. Chem. A. 2020, 124, 3564-3572] under troposphere conditions. The rate of the latter reaction was estimated to be 2 orders of magnitude slower than that of the N2O4 reaction under troposphere conditions.