Cryogenic liquids are being used as the active media for an increasing number of nonlinear optical devices. Results have been reported on stimulated Raman oscillators and amplifiers[1–4], third-harmonic generators, four-wave difference frequency generators[6, 7] and infrared Kerr switches. Photo-acoustic and photo-refractive measurements of weak absorptions in these liquids are also a subject of current interest [9, 10], In this paper, we report third-harmonic generation (THG) and ac Kerr effect measurements which allow us to obtain the electronic nonlinearity (hyperpolarizability), the vibrational two-photon resonance nonlinearity, and the molecular reorientation contributions to the third-order susceptibilities, x(3) of the cryogenic liquids CO, O2, N2 and Ar. Measurements of the electrostrictive and absorptive coupling of a laser radiation field to the liquid hydrodynamic modes are presented. These measurements are of interest because of their implications for the efficiencies and limitations of the nonlinear optical devices. Further, the relative simplicity of these media allow many of these quantities to be calculated or extrapolated from low density gas-phase measurements for comparison with these results. This should lead to an increased understanding of liquid-state properties.