The microwave spectrum of 2-propynylphosphine (propargylphosphine); H-C dropC-CH2-PH2, has been investigated in the 18-26.5 and 32-48 GHz spectral regions sit about -50 degreesC. Two conformers with different orientation:of the phosphino-group, denoted conformer I-and conformer Il, respectively, were: assigned. Conformer I has a symmetry plane (C-s symmetry) with both hydrogen atoms of the phosphino group pointing toward the triple bond (C-C-P-H dihedral angles approximately 47 degrees from syn-periplanar (0 degrees)). The C-C-P-H dihedral angles are 73 and 167 degrees, respectively, from syn-periplanar in,conformer II.,Only one of the hydrogen atoms of the phosphino group points toward the triple bond in this rotamer. Conformer I is 1.5(20) kJ/mol more stable than II. The dipole moment of LI was determined to be tin units of 10(-30) C m) mu (a) = 0 (assumed), mu (b) = 3.05(7), mu (c) = 1.60(9), and mu (tot) = 3.44(9) [mu (tot) = 1.03(3) D]. Two vibrationally excited states were assigned for each of the two rotamers I and II. Their frequencies were determined by relative intensity measurements. Many of the transitions of conformer II were split into two components presumably because of tunneling of the phosphino group. The tunneling frequency was determined to-be 0.814(42) MHz for the ground vibrational state and 11.49(18) MHz for the first excited state of the C-P torsional vibration. Quantum chemical calculations at the B3LYP and MP2 levels of theory using the 6-311++G(3df,2pd) basis set reproduced experimental rotational constants, quartic centrifugal distortion constants, and dipole moment components within a few percent. The energy difference between the two conformers was calculated using the Gaussian-2 theory, and conformer I was found to be more stable than conformer II by 2.1 kJ/mol.