The interactions of ethylene and its monohalogen-substituted derivatives with CO2 were investigated. Interaction energies of 16 stable complexes obtained range ftom -1.13 to -5.57 kJ/mol at the CCSD(T)/aug-cc-pVDZ//MP2/augcc-pVDZ level. The stability of the C2H4...CO2 complex is determined by the n...n interaction which is due to electron density transfer ftom MO-n(C=C) to MO-n(C=O) dominating the transition ftom MO-n(C=O) to MO-n(C=C). Remarkably, contribution of the n...n interaction to strength of complex of CO2-philic compounds and CO2 has been revealed for the first time. Substitution of an H atom by a halogen in C2H4 makes CO2, ,C2H3X (X = F, Cl, Br) slightly more stable than CO2...C2H4, and all the halogenated derivatives have equivalent stability. The strength of CO2...C2H3X which is contributed by Lewis acid-base interaction, attractive electrostatic interaction and hydrogen bond, is different nom the case of CO2...C2H4. The obtained results also show that, upon complexation, a contraction of C-H bond length involved in hydrogen bond and an increase in its stretching ftequency are affected by shape of the complex beside predominant dependence on polarity of C-H bond in isolated monomer.