The results are presented of a theoretical study of the nitrosation chemistry of pyrroline 1 (X = CH2), imidazoline 2 (X = NH), and 2-oxazoline 3 (X = O). Imines 1-3 are converted to the α-hydroxy-N- nitrosamines 7-9 via the N-nitrosoiminium ions 4-6. The NN-cis isomers of 7-9 may undergo retro-ene reactions to the δ-oxoalkyl diazotic acids 10-12. With the opportunity for microsolvation, C-X cleavage becomes possible for 8 and 9 and leads to the formation of N-(2-aminoethyl)- and N-(2-hydroxyethyl)-N- nitrosoformamides 15 and 16, respectively. The NN-isomerization barriers are comparable to the barriers for the ring-opening reactions, and the consideration of two Curtin-Hammett scenarios is required: CH-I for the NN-trans-rotamers of 7-9 to undergo C-X cleavage or NN-isomerization and CH-II for the NN-cis-rotamers to undergo C-X cleavage, C-N cleavage, or NN-isomerization. We determined all stereoisomers of the substrates, the products, and of all transition states structures for the retro-ene reactions of 7-9, the C-X cleavages of microsolvated 8 and 9, and the NN-isomerizations of 8 and 9. The potential energy surfaces were explored at the B3LYP/6-31G** level, and the results are discussed with emphasis on the comparison of the kinetics and thermodynamics of C-N versus C-X cleavage. The study shows all decompositions to be very fast with activation barriers below 21 kcal·mol-1, and the comparitive analysis predicts that the chemical toxicologies of 1 and 3 should be similar and remarkably different from that of 2.