The first gas-phase structure of a sultone is reported. Surprisingly, the four-membered ring of CF2CF2OSO2 is planar. The following ring parameters (r(a) distances and angle(alpha) angles with 3sigma uncertainties) were derived in the electron diffraction analysis : C-O = 1.41(2) angstrom, C-C = 1.545(15) angstrom, S-O = 1.656(4) angstrom, S-C = 1.866(6) angstrom, S-C-C = 85.8(10)degrees, C-C-O = 97.4(24)degrees, C-O-S = 98.9(16)degrees, and O-C-S = 78.0(5)degrees. A comparison of these results with a previously estimated structural model will be discussed.