Benson’s group additivity method is widely used for predicting thermodynamic properties, including heats of formation. However, due to the lack of experimental data, some of the group values are missing or poorly known for partially saturated heterocycles containing nitrogen. It has been shown that the energetics derived from quantum mechanical calculations can be used to predict thermodynamic quantities. In this work, the G2 method was used to calculate the heats of formation for 18 imines and related molecules (less than or equal to 4 non-H atoms). These values were then used to determine the group contributions for the missing imino-related groups, extending the applicability of Benson’s method. Very good agreement to experimental values was found for 2-alkyl-2-oxazolines (R = CH3 and CH2CH3). This approach was also compared to full G2 calculations for 1,4,5,6 -tetrahydropyrimidine. It was found that an unusually high ring strain term of 6.5 kcal/mol is required for Benson’s method to agree with the full G2 result (18.9 kcal/mol). Comparisons were made between the C-im-X groups estimated in this work and the previously established C-d-X group values. The values were quite different, indicating that the separate group distinctions should remain.