Crystals of difluorochlorine(III) hexafluoroantimonate(V) are triclinic, space group P, a= 5·60, b= 10·55, c= 5·30 Å; α= 92·1, β= 91·8, γ= 91·5°. The structure was refined by three-dimensional least-squares methods, to R 0·096 for 625 independent reflections. The chlorine atom has two nearest fluorine neighbours at 1·58 Å giving the expected angular ion, and two other neighbours at 2·33 and 2·43 Å, completing a much distorted square plane around chlorine. The two fluorine atoms which are more distant from the chlorine atom form part of the octahedral co-ordination of the antimony atoms and form trans-bridges about antimony, completing an endlesschain arrangement.