Ab initio SCF MO calculations of the force fields, normal vibrations, and dipole moment derivatives of acetamide and N-methylacetamide were carried out using the 4–31 G basis set for in-plane models and the 4–31 G and 4–31 G ∗ basis sets for out-of-plane modes. The absolute values of the calculated normal frequencies were 10–15% higher than the observed fundamental frequencies, which is the common trend of such a calculation with the 4–31 G basis set under the assumption of harmonic vibration. However, vibrational modes, isotope shifts of vibrational frequencies, and infrared intensities were reproduced quite well. Comparing the force constants of a series of amides, i.e., formamide [Y. Sugawara, Y. Hamada, A. Y. Hirakawa, M. Tsuboi, S. Kato, and K. Morokuma, Chem. Phys. 50, 105 (1980)], N-methylformamide [Y. Sugawara, A. Y. Hirakawa, M. Tsuboi, S. Kato, and K. Morokuma, Chem. Phys. 62, 339 (1981)], acetamide, and N-methylacetamide, systematic differences were found in the CONH moieties. On the other hand, the force constants of the CCH 3 and NCH 3 moieties were shown to be transferable among these amides.