The apparently conflicting results for the ionization energy of SF50 and the standard enthalpy of formation of SF5+ reported over the last 25 years have been reinterpreted and brought into line in terms of the existence of square pyramidal (C-4v) and trigonal bipyramidal (D-3h) isomers of SF50 and SF5+. New experimental results and theoretical calculations are reported which are consistent with this interpretation. Values are recommended for the ionization energy and standard enthalpies of formation for the two isomers of SF50 and SF5+.