The relatively thermally stable adduct Cl3AsO . AsF5 1 is prepared in high yield from AsCl4+AsF6- and Me(3)SiOSiMe(3) in liquid SO2 at -40 degrees C, and originally surprisingly isolated from the reaction of PrCl3 and AgAsF6 in liquid SO2; the X-ray crystal structure of 1 is compared with that of Cl3PO . AsF5.