The infrared spectra of gaseous and variable-temperature liquid xenon solutions of pyrrolidine have been recorded. The enthalpy difference has been determined to be 109 +/- 11 cm(-1) (1.30 +/- 0.13 kJ mol(-1)) with the envelope-equatorial conformer more stable than the twist form with 37 +/- 3% present at ambient temperature. Ab initio calculations utilizing various basis sets up to MP2(full)/aug-ccpVTZ have been used to predict the conformational stabilities, energy at the equatorialaxial saddle point, and barriers to planarity. From previously reported microwave rotational constants along with MP2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters have been obtained for both conformers. Heavy atom distances (angstrom) of equatorial[twist] conformer are as follows: N-1-C-2 = 1.469(3)[1.476(3)], N-1-C-3 = 1.469(3)[1.479(3)], C-2-C-4 = 1.541(3)[1.556(3)], C-3-C-5 = 1.541(3)[1.544(3)], C-4-C-5 = 1.556(3)[1.543(3)]; and angles (deg)