The deuterium kinetic isotope effects for rotations about the C-N bonds of formamide, N-methylformamide, and N,N-dimethylformamide have been calculated from, the Bigeleisen equation using the program QUIVER, based upon ab initio harmonic vibrational frequencies for ground states and transition states, calculated with GAUSSIAN 92 at the RHF/6-31G*, RHF/6-311++G**, MP2(FC)/6-31G*, and MP2(FC)/6-311++G** levels. The predicted isotope effects are similar at the different levels and agree closely with experimental values from Perrin et al. (J. Am. Chem. Soc. 1992, 114, 8800). The magnitudes of the isotope effects are interpreted.