The chemistry of a series of aziridines and diaziridines, including the parent compounds aziridine and diaziridine and several methyl-substituted analogues, is studied by high-level ab initio methods. The electronic structure methods employed include second-order Moller-Plesset perturbation theory (MP2), the coupled cluster singles and doubles method with a perturbative correction for connected triple substitutions [CCSD(T)], and the Brueckner doubles method with analogous corrections for triple and quadruple substitutions [BD(TQ)]. Computations are carried out with basis sets ranging in size and quality from 6-31C* and cc-pVDZ to cc-pV5Z. Stationary points for nitrogen inversion are located on the ground-state potential energy surfaces, and high-level barriers to nitrogen inversion are obtained. Thermodynamic activation parameters and deuterium isotope effects are computed for nitrogen inversion and compared with experimental results. The energetics are investigated for the conrotatory ring opening of aziridine, which produces an immonium ylide. Finally, accurate values are determined for the heat of formation of aziridine and diaziridine by application of high-level focal-point thermochemical analyses.