A process for preparing (per)fluorohalogenethers containing the --SO.sub.2F group, having general formula (I): FSO.sub.2--R--CF.sub.2OCAF--CA'F.sub.2 (I) wherein: A and A', equal to or different from each other, are Cl or Br; R has the following meanings: a (per)fluorinated, preferably perfluorinated, substituent, optionally containing one or more oxygen atoms; by reaction of carbonyl compounds having formula (II): FSO.sub.2--R--COF (II) wherein R is as above; in liquid phase with elemental fluorine and with olefinic compounds having formula (III): CAF.dbd.CA'F (III) wherein A and A' are as above, operating at temperatures from -120.degree. C. to -20.degree. C., optionally in the presence of a solvent inert under the reaction conditions.