Variable temperature (−60 to −100°C) studies of the infrared spectra (3500-400 cm −1) of ethylphosphine, CH 3CH 2PH 2, dissolved in liquid xenon have been recorded. From these data, the enthalpy difference between the more stable trans form and high energy gauche conformer has been determined to be 160 ± 30 cm −1 (457 ± 86 cal mol −1). From this enthalpy value along with the torsional transitions for both conformers and the gauche dihedral angle the potential function governing the conformer interconversion has been determined. Optimized structural parameters, conformational stabilities, barriers to internal rotation, harmonic force fields, infrared intensities, and scaled and unscaled fundamental frequencies were calculated for both conformers from MP2/6-31G∗ ab initio calculations. Raman line intensities and depolarization values were obtained from gradient RHF/6-31G∗ calculations. All of the results are compared with the experimentally determined values. Additionally, the results are compared with those obtained for ethyldifluorophosphine and ethyldichlorophosphine as well as to the corresponding isoelectronic molecules, methoxydifluorophosphine and methoxydichlorophosphine, respectively.