A selected ion flow tube apparatus was used to investigate the positive ion chemistry of two derivatives of SF6; SF5CF3 and SF5Cl. This represents the first investigation of the positive ion chemistry of SF5Cl, and much of the data on SF5CF3 is being presented here for the first time. Rate coefficients and ion product branching ratios have been determined at room temperature (300 K) for reactions with the following twenty-two cations; Ne+, F+, Ar+, N2+, N+, CO+, CO2+, O+, N2O+, H2O+, O2+, SF4+, CF2+, SF+, SF2+, NO2+, SF5+, NO+, CF+, CF3+, SF3+, and H3O+ (listed in order of decreasing recombination energy). Comparisons are made in the text for the reactions of these ions with SF6. SF2+, NO2+, NO+, SF3+, and H3O+ are found to be unreactive with both of the derivatives. The majority of the other reactions proceed with rate coefficients which are close to the capture value. The exceptions are the reactions of O2+, SF+, SF5+, and CF3+ with SF5CF3, and SF4+and SF5+ with SF5Cl, all of which have rate coefficients significantly less than the capture mechanism value. Several distinct processes are observed among the large number of reactions studied, including dissociative charge transfer and various abstraction channels. Non-dissociative charge transfer is not observed, implying that any parent ions formed dissociate rapidly to the fragment ions and associated neutrals.