The formation of weakly bound molecular complexes between methylenecyclopropane (MeCP) and BF3 dissolved in liquid argon has been investigated using FTIR spectroscopy. Experimental evidence was found for the formation of a 1:1 complex in which the BF, moiety binds to the exocyclic C=C double bond, whereas at higher concentrations of BF3, weak absorption bands due to MeCP (BF3)(2) were also observed. Using spectra recorded at different temperatures between 105 and 129 K, the complexation enthalpy for the 1:1 complex in liquid argon was determined to be -10.7(3) kJ mol(-1). Structural and spectral information on the complexes was obtained from density functional calculations at the B3LYP/6-311++G(d,p) level. Applying Free Energy Perturbation Monte Carlo calculations to account for the solvent influences and statistical thermodynamics to calculate the zero-point vibrational and thermal influences, the complexation energy for the 1:1 complex was estimated, from the experimental complexation enthalpy, to be -16.0(10)kJ mol(-1). This value is compared with the B3LYP/6-311++G(d,p) complexation energies and with single point energies calculated at the MP2 = full/aug-cc-PVTZ level.