Adducts between BiF5 and SbF5 were prepared in liquid WF6 at room temperature. Sublimation of the product gave various volatile crystalline products with analyses corresponding to BiF5(SbF5)3, BiF5(SbF5)2, BiF5(SbF5)1.5, and (BiF5)nSbF5(n= 2, 3, or 20.6). The adducts (BiF5)nSbF5(n= 1 or 1.5), were also prepared. The BiF5(SbF5)n(n= 1.5, 2, or 3) adducts were of low melting point and of higher volatility than (BiF5)nSbF5(n 1). Single-crystal X-ray diffraction studies showed that BiF5(SbF5)n(n= 2 or 3) were isomorphous with tetrameric (NbF5)4, and an X-ray powder diffraction photograph of BiF5(SbF5)1. was very similar to that of BiF5(SbF5)n(n= 2 or 3), indicating that it was also isostructural with (NbF5)4. The BiF5(SbF5)3 structure likely consists of disordered BiF5(SbF5)3 tetramers, and BiF5(SbF5)2 likely consists of a mixture of disordered BiF5(SbF5)3 and (BiF5.SbF5)2 tetramers in the appropriate ratios. The molecular-beam mass spectra of SbF5(BiF5)n(n= 2 or 3) show fragment ions attributable to BiF5 and SbF5 as well as various associated pentafluoride clusters, including BiF5(SbF5)3 for the n= 3 adduct. The vibrational spectra of BiF5(SbF5)n(n= 1.5, 2, or 3) were similar and indicative of the presence of SbF5 and BiF5 units joined by cis-bridged fluorine atoms. (BiF5)nSbF5(n= 1, 1.5, 2, 3, or 20.6), were shown by X-ray powder diffraction photography to be isomorphous with polymeric trans-bridged BiF5 and therefore to consist of disordered BiF5 and SnF5 units linked by trans-bridged fluorine atoms. The trans-bridged polymeric BiF5-type structure of the adducts was further supported by their vibrational spectra. The behaviour of BiF5-(SbF5)n(n= 2 or 3) in WF6 solution is discussed. An estimated phase diagram for the system BiF5–SbF5 was constructed.