The potential energy surfaces (PESs) of H(2)CXH(n)(+) (X = O, n = 1; X = N, n = 2; X = S, n = 1) systems were investigated by using the ab initio method at MP2/6-311+G** and QCISD/6-311+G** levels, and single-point calculations on the MP2 and the QCISD geometries were performed at the QCISD(T)/6-311+G(3df,2p) level. At the QCISD(T)/6-311+G(3df,2p)//MP2/6-311+G** level including ZPVE correction from MP2/6-311+G** unsealed frequencies, the proton affinities are 168.9, 207.2, and 182.9 kcal/mol for formaldehyde, formaldimine, and thioformaldehyde, respectively, while the energy barriers and reaction energies for hydrogen 1,2-elimination from the corresponding protonated systems are 79.3 and 22.0, 90.3 and 42.2, and 61.9 and 27.9 kcal/mol, respectively. Hydrogen 1,l-eliminations are not competitive compared with 1,2-eliminations. The reaction path for hydrogen 1,2-elimination from protonated formaldehyde goes through planar structures, whereas those from protonated formaldimine and thioformaldehyde involve rotation about the C-X double bond. The hydrogen 1,2-elimination takes place through a concerted mechanism for protonated formaldehyde and formaldimine, whereas for protonated thioformaldehyde a two-step mechanism was found on the electronic PES. By taking account of the ZPVE correction, a concerted mechanism results. For protonated thioformaldehyde a TS for proton scrambling was found about 5 kcal/mol more stable than the TS for hydrogen 1,2-elimination (this energy difference remains even at the QCISD(T)/6-311+G(3df,2p)//MP2/6-311+G(3df,2p) level). This result is in contrast with deuterium label experiments.