When pentafluoropyridine and heptafluoro-quinoline and -isoquinoline react with hydrogen halides in tetrahydro-thiophen dioxide, replacement of fluorine ortho and para to ring nitrogen by the other halogen occurs. The order of reactivity is heptafluoroquinoline ⋙ heptafluoroisoquinoline > pentafluoropyridine. Reaction with heptafluoro-quinoline occurs at room temperature with substitution first at the 2- and then at the 4-position, and good yields of the 2,4-dihalogeno-derivatives can be obtained directly. Elevated temperatures are required for reactions with pentafluoropyridine and heptafluoroisoquinoline; pentafluoropyridine gives the 4- and 2,4,6-substituted derivatives and heptafluoroisoquinoline the 1-substituted derivative, in low yields. The presence of small amounts of water in the solvent is found to deactivate the system. The mechanism of these reactions is discussed in terms of nucleophilic displacement of fluoride ion from the protonated species.