The infrared (3500 to 30 cm(-1)) spectra of gaseous and solid and the Raman (3500 to 20 cm(-1)) spectra of the liquid [with quantitative depolarization ratios] and solid cyclopropylmethylacetylene, c-C3H5CH2CCH (ethynylmethylcyclopropane), have been recorded. Additionally, variable temperature (-55 to -100 degrees C) studies of the infrared spectra (3500 to 400 cm(-1)) of the title compound dissolved in liquid xenon have been carried out. Utilizing four conformer pairs, an enthalpy difference of 147 +/- 14 cm(-1) (1.76 +/- 0.17 kJ/mol) was obtained, with the cis rotamer the more stable conformer. At ambient temperature there is approximately 48% of the cis conformer present. However, the gauche conformer remains in the annealed solid. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies are reported for both conformers from ab initio calculations. The calculations were carried out at the restricted Hartree-Fock (RHF) level and/or with full electron correlation to second order (MP2), utilizing the following basis sets: 6-31G(d), 6-311G(d), 6-311++G(d,p) and 6-311+G(2d,2p). From the RHF calculations (6-31G(d)) the gauche form was predicted to be the more stable rotamer by 133 cm(-1) (1.59 kJ/mol), but from all of the calculations with electron correlation, the cia form is predicted to be the more stable rotamer and from the largest basis set by 174 cm(-1) (2.08 kJ/mol), which is consistent with the experimental results. The other calculated quantities are compared to the experimentally determined values where applicable, as well as with some corresponding results for some similar molecules.