The gas phase reactions of the substituted methyl cations, XCH 2 + (X = F, HO, and Cl), with formamide were investigated using ab initio calculations. Addition of the carbocations at the nitrogen atom of the amide bond yields “N adducts” that are less stable than the “O adducts” arising from attack at the carbonyl oxygen atoms. The subsequent fragmentation reactions of these adducts were examined including: (1) fragmentation of the “O adduct” by elimination of HX via a transition structure involving 1, 6 H + transfer, resulting in the formation of HC(OCH 2 )NH + ; (2) fragmentation of the “N adduct” by elimination of HX via a transition structure involving 1, 3 H + transfer resulting in the formation of HC(O)NHCH 2 + ; (3) fragmentation of the “N adduct” via a transition structure involving 1, 3 HC(O) + transfer resulting in the formation of HC(O)XCH 2 NH 2 + , which decomposes to yield HC(O)X and CH 2 NH 2 + (when X = OH and Cl); (4) fragmentation of the “N adduct” via extrusion of CO (when X = F and Cl).