The N- and O-site protonation affinities for formamide and thioformamide were studies by ab initio molecular orbital calculations using the 6-31G** basis set at the Hartree-Fock and up to the MP4 level. The proton affinity for O-site is found to be more favorable than that for the N-site by 13-26 kcal mol(-1), dependent on the level of calculation. The natural bond orbital analysis shows that the O-site protonation can enhance the zwitterionic contribution and thus increase the rotational barrier around the C-N bond. The N-site protonation has the opposite effect. For an O-site protonation, both sigma- and pi-orientation were considered. We also investigated a weaker interaction of the hydrogen bond at the two sites with the HF molecule as a hydrogen donor. We made a comparison between formamide and thioformamide.