Complexes of mixed boron halides BFxCl3-x (x = 1, 2) with CH3F have been detected in solutions in liquid argon (86-115 K), using infrared spectroscopy. The spectra of the complexes have been interpreted by using ab initio calculations at the MP2/6-31+G** level, and indicate the presence of a conformational equilibrium for both boron halides. For CH3F . BF2Cl the conformationally averaged Delta H degrees was measured to be -12.4(3) kJ mol(-1), while for CH3F . BFCL2 it was found to be -10.8(6) kJ mol(-1) for the C-l conformer, and -8.6(6) kJ mol(-1) for the C-s conformer. For solutions containing BCl3, even at the highest concentrations, no adducts with CH3F were detected. Combined with existing data on CH3F . BF3, these results show that for weakly bound complexes the Lewis acidity of the boron halides contradicts the established order. This is attributed to the electrostatic nature of the van der Waals bond.