An addition product which is very likely a,Q-sultone vas obtained from the spontaneous reaction between chlorotrifluoroethylene and sulfur trioxide as a distillable colorless liquid (Ia+Ib; b.p.76.0-76.5°,n_D^20° 1.3672, d^21.8° 1.71).Its structure was elucidated by elementary analysis, determinations of molecular weight and saponification equivalent, and its infrared spectrum.Preliminary studies of its chemical behavior are also here reported.