The structure and conformational composition for 3,4-epoxy-1-butene have been studied by ab initio molecular orbital calculations at the HF/6-31G* level and a combined least-squares analysis of electron diffraction data and microwave rotational constants. Ab initio calculations showed the presence of three stable conformers, anti, gauche-1, and gauche-2 with the anti form being lowest in energy. The gauche-l and gauche-2 forms were 0.51 and 2.32 kcal mol(-1) higher in energy, respectively. Using calculated values for entropies and zero-point and electronic energies of these three forms, a conformational mixture of 76% anti, 22% gauche-1, and 2% gauche-2 was obtained at the experimental temperature of 298 K. The combined electron diffraction and microwave data analysis indicated the presence of both the anti and gauche-l forms, with a preference for the anti form [64(17)%]. For this model, the experimental values for the geometrical parameters (r(g), angle(alpha)) are as follows (values given for the anti form): r(C-H)(av) = 1.097(5) Angstrom, r(C-4=C-5) = 1.331(4) Angstrom, r(C-3-O-1) = 1.436(2) Angstrom, r(C-2-O-1) = 1.429(2) Angstrom, r(C-2-C-3) 1.477(7) Angstrom, r(C-3-C-4) 1.500(7) Angstrom, angle C4C3O1 = 115.2 degrees(19), angle C-5=C4C3 = 123.5 degrees(21), phi CCC=C = 146(12)degrees.