The decarbonylation reactions of ionized 2-acetylpyridine, 2-acetylpyrazine, and 2-acetylthiazole have been investigated using the multiple collision technique of neutralization–reionization/collision-induced dissociation mass spectrometry and related techniques. The resultant heterocyclic C6H7N·+, C5H6N2·+, and C4H5NS·+ ions were identified as 2-methylene-1,2-dihydropyridine, 6·+, 2-methylene-1,2-dihydropyrazine, 9·+, and 2-methylene-2,3-dihydrothiazole, 12·+, respectively. This result refutes proposals in the older literature that the decarbonylation would involve a methyl transfer yielding the 2-methyl species 2-methylpyridine (2·+), 2-methylpyrazine (8·+), and 2-methylthiazole (11·+). Literature proposals for a methyl transfer in the dissociation of ionized dimethyl-2,3-pyridinedicarboxylate and methyl-4-pyridinecarboxylate were also examined, but could not be substantiated either. However, the proposed gas phase synthesis of the N-methylpyridinium ylide, 1·+, from ionized methyl-2-pyridinethiocarboxylate did provide evidence for a genuine methyl migration. To reinforce these conclusions, the dissociation characteristics of isomers structurally related to 6·+ (3·+– 5·+ and 7·+), to 9·+ (10·+) and to 12·+ (13·+– 15·+) were also considered. Exploratory quantum chemical calculations (at the B3LYP/6-31G∗ level of theory) indicate that 6·+ and 12·+ are among the most stable isomers in the C6H7N·+ and C4H5NS·+ systems, lying 24 and 19 kcal/mol lower in energy than 2·+ and 11·+, respectively. Their neutral counterparts, however, are considerably less stable: 6 is calculated to be 27 kcal/mol higher in energy than 2. Nevertheless, from neutralization–reionization experiments it follows that the neutral counterparts of the ionized decarbonylation products 6, 9, and 12 are stable molecules on the microsecond time scale. Significant 1,3-hydrogen shift barriers hinder the interconversion of both the ions and the neutrals into their 2-methyl substituted counterparts, thus accounting for their stability in the dilute gas phase.