The spontaneous reactions of N2O4 (NTO) with hydrazines (XNH2, X = NH2, CH3NH and (CH3)2N, denoted as HZs) have been studied by ab initio quantum chemical calculations at the PCM-CCSD(T)/6-311+G(3df,2p) level in CCl4 solution at low temperature to elucidate the mechanisms and predict the kinetics of these well-known hypergolic processes experimentally studied by Saad et al. [AIAA J. 10 (1972) 1073; Ref. 1]. The key initiation reactions in these systems, similar to those unraveled recently by us for the gas-phase reactions, occur by the isomerization process of N2O4 → ONONO2 via very loose transition states within the pre-reaction complexes (N2O4:XNH2) with barriers from 7.2 to 8.7 kcal/mol, followed by concurrent ONONO2 attack on XNH2 through abstraction of one of H atoms by the NO3 group to form HNO3 + XN(H)NO. The predicted rate constants for these bimolecular reactions in CCl4 at 253 K were primarily controlled by transformation of N2O4:XNH2 to ONONO2:XNH2 within 4.7–278 ms. This result can satisfactorily account for the occurrence of the hypergolic reactions of NTO and HZs in CCl4 solution at 253 K reported by Saad et al.