Perfluoroalkylbenzenes under a treatment of a system Zn(Cu)-DMF-H2O-electrolyte give rise in high yield to products of fluorine substitution by hydrogen in position 4. In perfluoro-tert-butylbenzene are substituted by hydrogen the fluorine atoms in positions 2 and 4; here the DMF-water ratio affects the regioselectivity of the process. In the perfluoro-4-tert-butyltoluene is mainly substituted the fluorine in the ortho-position to the C(CF3)(3) group.