The mid-infrared (4000-400 cm(-1)) spectra of ethene/BF3 and propene/BF3 mixtures, dissolved in liquid argon (93-125K) and in liquid nitrogen (80-118K) are discussed. In all spectra, the formation of a 1:1 van der Waals complex, in which the electron deficient BS molecule binds to the C=C double bond, was observed. Using spectra recorded at different temperatures, the complexation enthalpy Delta H degrees for ethene BS was determined to be -10.0 +/- 0.2 kJ mol(-1) in liquid argon and -5.4 +/- 0.3 kJ mol(-1) in liquid nitrogen, while for propene . B-3 the Delta H degrees are -11.8 +/- 0.2 and -6.9 +/- 0.3 kJ mol(-1), respectively. For both complexes a converged structure and the harmonic vibrational frequencies were calculated using ab initio calculations at the MP2/6-31+G* level.