Abstract: Process for preparing perfluoropolyethers with bromo- or iodo- containing end groups, respectively, by subjecting a peroxidic perfluoropolyether to thermal treatment, in the absence of UV radiation, at temperatures from 120.degree. C. to 280.degree. C. in the presence of bromo- or iodo-containing compounds having, respectively, the following general formulas:T T' T" C Brwherein T, T', T" equal to or different from each other are selected from Br, F, H or R'.sub.f, wherein R'.sub.f is a perfluoroalkylic chain from 1 to 8 carbon atoms;R R' R" C Iwherein R, R', R", equal to or different from each other can be I (iodine), H, F, or R'.sub.f, wherein R'.sub.f has the meaning indicated above.