When reacted with an excess of the corresponding carbonyl halides, AsF5 and SbF5 form the following 1:1 adducts: COCl2. AsF5, COCl2. SbF5, COClF . AsF5, COClF . SbF5, COF2. AsF5, and COF2. SbF5. All adducts are unstable at ambient temperature, and their dissociation enthalpies were determined from the dissociation pressure curves. Vibrational and multinuclear NMR spectra and theoretical calculations show that all compounds are oxygen-coordinated donor-acceptor adducts, and that the strengths of the oxygen bridges increase from COF2 to COCl2 and from AsF5 to SbF5. Full normal coordinate analyses of the adducts demonstrate that the bridging modes occur below 100 cm(-1), justifying the frequently used approximation of analyzing similar weak adducts in terms of their separate donor and acceptor molecules.