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The insertion of CO2 into epoxides and aziridines has been studied using density functional theory (B3LYP) and ab initio (MP2) methods, and the effect of substitution for the two reactions are further explored. It is found that the reactivity of epoxides and aziridines are similar, and insertion of CO2 proceeds through a concerted mechanism. The substitutions of methyl and phenyl does not change the reaction mechanism, but the transition state for the substitution on the attacking position becomes loose with a lower free energy barrier. The substitutions of methyl and phenyl decrease the free energy barrier, with phenyl substitution having a greater affect. The results also show that the free energy barriers for the insertions of CO2 into aziridines are ~10 kcal mol−1 lower than the corresponding reactions of CO2 with epoxides. The insertion of CO2 into epoxides and aziridines have been studied using B3LYP and MP2 methods, and the effect of substitution for the two reactions are further explored. Mechanistic details of the reactions of CO2 with epoxides and aziridines can provide meaningful information for the reutilisation of CO2. [ABSTRACT FROM AUTHOR]