Abstract: The gas-phase structure of ethylene primary ozonide (CH2CH2000) has been determined from millimeter wave spectra of five isotopic species. Partial substitution, r,, parameters for the lowest energy oxygen envelope conformation (C, symmetry) are r (CC)= 1.546 (3) A, r (C0)= 1.417 (10) A, r (O0)= 1.453 (10) A, r (CHcodo)= 1.088 (5) A, r (CHo,)=