Alternating current Kerr effect and third-harmonic generation (THG) are reported for the cryogenic liquids CO, O2, N2, and Ar pumped by CO2 TEA laser radiation. The THG experiments measure the hyperpolarizability, while the Kerr effect measurements probe the total nonresonant third-order susceptibility χ(3), arising from both molecular orientation and electronic hyperpolarizability. The THG process in liquid CO is dominated by a vibrational two-photon resonant contribution for the CO2 R(6) pump line at 9.35 μm. By studying the interference between the resonant and nonresonant contributions in liquid CO–O2 mixtures the hyperpolarizability of liquid O2 has been determined relative to the two-photon resonant χ(3) of liquid CO. These measurements give the first experimental confirmation for liquid media that the two-photon resonance contribution to polarized scattering consists of both narrow and broad linewidth components associated with, respectively, the isotropic and anisotropic Raman polarizabilities.