The inversion of 1,4-dioxane has been studied by using ab initio molecular orbital theory at the HF/6-31G* and BLYP/6-31G* levels. A total of 10 stationary points were characterized as energy minima or transition states. Vibrational frequencies were calculated for the chair and two twist-boat conformations. The chair conformation is the lowest in energy, followed by the two twist-boats. The transition state connecting the chair and the twist-boats is a half-chair structure, in which four atoms in the ring are planar. Thermochemical analysis of the chair, twist-boats, and the half-chair structures yields a value of the free energy of activation to ring inversion that is in good agreement with experiment.