Reactions SbF5 + nHF + PF3 (n = 1, 2) have been investigated using ab initio and DFT methods. This work was motivated by the recent experimental study of Seppelt and co-workers [Inorg. Chem. 39 (2000) 2784] who obtained new salt PF3H+ - SbF6- . HF as a product for n = 2. The calculated gas-phase cluster, HPF4 . SbF5-HF, resembles the solid state ion pair in geometry and charge distribution. We have also found a low-barrier isomerization between the equivalent structures of the distinct cluster, HPF4-HF . SbF5. It is suggested that this isomerization mechanism may be responsible for the exchange of the F atoms indicated for the SbF5/HF/PF3 system in HF solution [loc. cit]. (C) 2002 Elsevier Science B.V. All rights reserved.