The geometric structure and conformation of the satd. five-membered ring 4,4,5,5-tetrafluoro-1,3,2-dithiazolidine (I) have been detd. in the gas and solid phases by electron and x-ray diffraction, resp. The equil. structure and the total energy along the pseudorotational path have been calcd. by ab initio methods (HF/4-21G(*) and HF/TZP). In the gas phase, the five-membered ring adopts a near envelope conformation with one sulfur atom bent out of the CCSN plane and with the N-H bond in the axial direction. The ab initio calcns. predict this conformation to be the only stable structure along the pseudorotational path. Two mols. with different conformations are present in the crystal. Whereas one mol. adopts an envelope conformation which is very similar to the gas-phase structure, the other mol. possesses an envelope conformation with the nitrogen atom bent out of the SCCS plane. According to the ab initio calcns., the latter conformation corresponds to a distorted structure and is 1 kcal/mol higher in energy. The two mols. are linked by a nearly linear N...H-N hydrogen bond with N...H = 2.30 .ANG. and N...N = 3.099 .ANG..