The far infrared spectrum of 1-pentyne, CH3CH2CH2C=CH, has been recorded in the gas phase. The fundamental asymmetric torsional transitions have been observed at 114 and 109 cm(-1) for the gauche (synclinal) and trans (antiperiplanar; methyl group trans to the acetylenic group) conformers, respectively. The methyl torsional fundamental (249.8 cm(-1)) has only been observed for the gauche conformer. Infrared spectra (3500-400 cm(-1)) of 1-pentyne dissolved in liquid xenon have been recorded from which variable temperature (-60 to -100 degrees C) studies have been carried out. From these data, the enthalpy difference has been determined to be 113 +/- 26 cm(-1) (323 +/-74 cal/mol) with the trans conformer more stable than the gauche form. The Raman spectrum (3500-40 cm(-1)) has also been recorded of the liquid to aid in the assignment of the fundamentals. Ab initio electronic structure calculations of energies, conformational geometries, vibrational frequencies, and potential energy functions have been carried out to complement and assist in the interpretation of the spectra. In particular, the transitions among torsional energy levels for both the symmetric (methyl) and asymmetric (ethyl) motions have been calculated. The results are compared to the corresponding quantities for some similar molecules.