The infrared spectra (3500-50 cm(-1)) of gas and solid and the Raman spectra (3500-50 cm-1) of liquid and solid 1-penten-4-yne, CH2=CHCH(2)Cequivalent toCH, have been recorded. Variable-temperature studies over the range -105 to -150 degreesC of the infrared spectra (3500-400 cm(-1)) of the sample dissolved in liquid krypton have also been recorded. By utilizing the relative intensities of five syn/gauche conformer pairs, the syn conformer is found to be the lower energy form with an enthalpy difference of 248 +/- 25 cm(-1) (2.97 +/- 0.30 kJ/mol). At ambient temperature it is estimated that there is 38% +/- 2% of the gauche conformer present. Equilibrium geometries and total energies of the two conformers have been determined from ab initio calculations with full electron correlation by the perturbation method to second order as well as by hybrid density functional theory (DFT) calculation with the B3LYP method using a number of basis sets. The smaller basis sets with diffuse functions predict the gauche conformer to be the more stable form, whereas other calculations predict the syn conformer to be the lower energy rotamer. The B3LYP calculations predict a much lower energy difference between the conformers than found experimentally. A complete vibrational assignment is proposed for the syn conformer and many of the fundamentals have been identified for the gauche form on the basis of the force constants, relative intensities, and depolarization ratios obtained from MP2/6-31(d) ab initio calculations as well as on rotational-vibrational band contours obtained from the predicted equilibrium geometric parameters. The r(0) structural parameters are estimated for 1-penten-4-yne from ab initio MP2/6-311 +G(d,p) calculations, and comparisons are made with those of allyl fluoride and cyanide. The spectroscopic and theoretical results are compared to the corresponding properties for some similar molecules.