Microwave assignments of the normal, monodeutero, dideutero, oxygen-18 and carbon-13 isotopic species of cis-1,2-difluoroethylene oxide have given a complete structure. The molecular parameters of the r8 structure are r (CO) = 1.409 (4) Å, r (CC) = 1.451 (3) Å, r (CH) = 1.093 (4) Å, r (CF) = 1.347 (7) Å, (HCF) = 111.0 (4)°, and (CCO) = 59.01 (20)°. A value of 2.89 (2) D was found for the electric dipole moment. Fluorines have the effect of shortening the adjacent bonds in this ethylene oxide derivative. It is found that a molecular orbital picture of the epoxide ring does not satisfactorily rationalize this shortening in the ring. The molecule provides another example of the apparent constancy of vicinal nonbonded fluorine-fluorine distances in fluorocarbons. An analysis of the HCF angle contraction relative to the HCH angle in ethylene oxide shows the CF bond orientation remains nearly the same while the CH bond moves toward the CF bond. This follows the trend found in a number of simple compounds containing the HCF group. Observation of this effect in cis- and trans-1,2-difluoroethylene does not seem to support the idea of an intramolecular bond between vicinal fluorine atoms.