[en] The fluoride-ion affinity (A/sub F^-/) of phosphorus pentafluoride was determined to be 100 kcal/mole from the heats of reaction of the Lewis bases SF₄ and ClO₂F with PF₅ near room temperature. The fluoride-ion affinity of boron trifluoride was determined to be 92 kcal/mole from the heat of reaction of ClO₂F with BF₃. The crystal structure of ClO₂BF₄ was determined and a precise lattice energy was calculated from this structure and used to determined A/sub F^-/. Both PF₅ and BF₃ were found to react with graphite in the presence of fluorine gas to yield a variety of non-stoichiometric compounds. The fluoride-ion affinity of silicon tetrafluoride is not known, but it does not react with graphite and F₂ except at high pressures. These and previous results suggested a threshold in oxidizing power of intercalating species below which the oxidative intercalation reaction would not occur. The reduction of C/sub x/PF₆ by PF₃ proved that the reaction is thermodynamically controlled to some extent. The displacement of PF₅ in C/sub x/PF₆ by BF₃ (with a smaller A/sub F^-/) suggested that two BF₃ molecules may have a larger fluoride-ion affinity than one PF₅ and that B₂F₇^- may be a stable anion in graphite. Conductivity studies of PF/sub x/ and BF/sub y/ salts showed that a large drop in conductivity when the reaction reaches first stage is due in the most part to direct fluorination of carbon in graphite