The mid-infrared spectra (3100-400 cm - 1 ) of ethyldimethylphosphine CH 3 CH 2 P(CH 3 ) 2 dissolved in liquid xenon as a function of temperature (- 60 to - 100 o C) have been recorded. Using the conformer pairs (trans/gauche) observed at 937/947 and 1006/1016 cm - 1 , the enthalpy difference has been determined to be 276 +/- 37 cm - 1 (790 +/- 105 cal mol - 1 ) with the gauche conformer being more stable than the high energy trans form. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for both conformers from RHF/6-31G * and MP2/6-31G * ab initio calculations. The ab initio calculations predict the gauche conformer to be more stable in agreement with the experimental results. The other calculated quantities are compared to the experimentally determined values when appropriate. These results are compared to those obtained for ethylphosphine, CH 3 CH 2 PH 2 , ethyldifluorophosphine, CH 3 CH 2 PF 2 , and ethyldichlorophosphine, CH 3 CH 2 PCl 2 where similarities and differences are documented and discussed.