Complex formation between boron trifluoride and either alkyl fluoride or olefin–hydrogen fluoride mixtures, has been investigated at –90° in the solvent methylene chloride, by observing the vapour pressure of boron trifluoride above the solutions. The results are difficult to interpret, but suggest that boron trifluoride is absorbed both as RF·BF₃ and as R′·F(BF₃)₂. The latter complex, in which R′ may be polymeric, is formed rapidly from olefins slowly from alkyl fluorides; olefins may be intermediates in the reaction of alkyl fluorides.