A comparative ab initio investigation of the geometries, force constants, and vibrational frequencies is presented for a series of saturated three-membered-ring compounds. The calculations have been performed with minimal, split-valence, and split-valence-polarized basis sets. For the spectral results we have found that the minimal basis set results are only of marginal valuesince they often predict force constants to be too large by over 40%. The errors in the off-diagonal force constants further cause the order of the predicted modes to be grossly in error. At the split-valence and split-valence-polarized levels we have found that the calculations predict harmonic frequencies to be too high by about 10-15%. On the basis of our calculations we propose several revisionsin the currently accepted force field for both cyclopropane and ethylene oxide. Our calculations suggest what we believe to be the most complete force field for ethylene imine. This is especially important since previous experimental refinements have neglected a number of interaction force constants which we show to be important in the correct experimental assignment of thespectra. A scaling of the diagonal elements of the force constant matrix produces reasonably good agreement with experiment. We have also shown how an analysis of the normal modes provides a very useful aid in the interpretation and assignment of experimental spectra, especially in cases where a number of internal coordinates contribute to a molecular vibration. A comparison of the normal modes for cyclopropane, ethylene oxide, and ethylene imine points out the similarity among these compounds and provides a foundation for a functional-group approach is interpreting their infrared spectra. For completeness we also include energies and dipole moments evaluated at the SCF level with a near Hartree-Fock basis.