The mid infrared spectra of methylcyclopropane-HCl and methylcyclopropane-BF3 mixtures, dissolved in liquid argon (LAr) or liquid nitrogen (LN2), have been examined. In LAr solutions, evidence was found for the formation of two different isomers of the 1:1 van der Waals complexes-an asymmetric complex, in which the electron acceptor interacts with the C-C bond adjacent to the methyl. group, and a symmetric complex in which the interaction occurs with the opposite C-C bond. At higher concentrations, indications were found for the formation of a chain-type 1:2 complex. In LN2 solutions, only the two 1:1 isomers, of the HCl complex have been observed. Using spectra recorded at different temperatures, the complexation. enthalpy AW in LAr was determined to be -9.5(4) and -8.3(3) kJ mol(-1) for the symmetric and asymmetric MCP-HCl complexes, and -9.1(2) and -6.7(5) kJ mol(-1) for the symmetric and asymmetric MCP.BF3 complexes, respectively. In addition, for all investigated species, fall MP2/6-31+G(d,p) geometry optimizations and B3LYP/6-311++G(d,p) frequency calculations were performed, and the complexation. energies were calculated at the MP2/6-311++G(3df,2pd) level. These calculations indicate that the isomers of the MCP-HCl complex are of comparable strength, while the asymmetric MCP.BF3 complex is significantly less stable than its symmetric counterpart. Using Free Energy Perturbation Monte Carlo simulations to calculate the. solvent influences, and using statistical thermodynamics to account for zero-point vibrational and thermal contributions, from the experimental complexation. enthalpies, the complexation energies are estimated to be - 15.7(8) and -16.0(9) kJ mol(-1) for the symmetric and asymmetric MCP.HCl complexes, which is in agreement with the. ab initio results, and -14.0(8) and -15.3(5) kJ mol(-1) for the corresponding MCP.BF3 species.