The reactions of antimony and bismuth trifluorides with the pentafluorides of antimony and arsenic in liquid sulfur dioxide yield the adducts SbF3•SbF5(type A), SbF3•AsF5, BiF3•3SbF5, and BiF3•AsF5; no adducts of the trifluorides with BF3were isolated. Antimony metal reacts with excess of antimony pentafluoride and arsenic pentafluoride in liquid sulfur dioxide giving a second form of SbF3•SbF5(type B) and the same form of SbF3•AsF5as produced from SbF3, respectively. BiF3•3SbF5can be formulated as Bi(SbF6)3, but its Raman spectrum indicates the presence of extensive cation–anion fluorine-bridging. Their Raman spectra suggest that BiF3•AsF5, SbF3•AsF5, and SbF3•SbF5(type B) have similar structures, possibly containing polymeric fluorine-bridged cations, but that SbF3•SbF5(type A) has a different structure probably like that of AsF3•SbF5. The121Sb Mössbauer spectra of the SbF3adducts support the assignment of similar structures to SbF3•AsF5and SbF3•SbF5(type B), and the suggestion that SbF3•AsF5and both forms of SbF3•SbF5contain antimony (III) fluoro-cations.19F n.m.r. shows that SbF3has appreciable solubility in SO2solutions of SbF5and indicates the possible formation of "fluxional" SbF2–SbnF5n+1molecules in SbF3–SbF5solutions.