The infrared (3500-40cm - 1 ) spectra of gaseous, liquid and solid 3-methyl-3-butene nitrile, CH 2 C(CH 3 )CH 2 CN, and trans 3-pentenenitrile, CH 3 CHCHCH 2 CN, have been recorded. Both the cis and gauche conformers have been identified for both conformers in the fluid phases but only the cis form remains in the solid. Variable temperature (-55 to -100 o C) studies of the infrared spectra of the samples dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 174+/-20cm - 1 (2.08+/-0.24kJ/mol) for 3-methyl-3-butene nitrile with the cis conformer the more stable rotamer and 187+/-20cm - 1 (2.23+/-0.24kJ/mol) for trans 3-pentenenitrile, again with the cis rotamer the more stable form. Complete equilibrium geometries have been obtained for both rotamers for both molecules by ab initio calculations employing the 6-31G(d), 6-311G(d,p), 6-311+G(d,p) and 6-311+G(2d,2p) basis sets at the levels of restricted Hartree-Fock and/or Moller-Plesset to second order. Only the MP2/6-311+G(2d,2p) calculation gives the correct conformer stability for 3-methyl-3-butene nitrile.