Molecular-beam spectroscopic data on (HF)2 are used to determine the HF⋯HF intermolecular potential-energy surface. The method used in based on the BOARS approximation of Holmgren et al.(J. Chem. Phys., 1977, 67, 4414), which is extended to diatom–diatom weakly bound complexes. Molecular constants are calculated for several proposed potential-energy surfaces and compared with experiment. The optimised potential surface is shown to be similar to those derived from ab initio calculations. It is believed accurately to represent the shape of the true surface in the region of the potential minimum and along the path of the tunnelling motion; further information is required before an accurate well depth may be obtained.