A novel femtosecond IR method is introduced and used to study the third-order response of liquids. This method, based on the gated detection of a CW probe beam, requires only one ultrafast pulse and a CW probe beam. This opens the possibility for the study of the two-color third-order response in a much wider spectral range. It is demonstrated here that this detection method is sensitive to the change in both the amplitude and phase of the probe field. In transparent liquids, the third order susceptibility affects the geometry, polarization, and frequency of the probe IR beam, all of which can be measured by this method. Examples of study with optical femtosecond pump and CW IR probes on CS2 and other liquids are presented. The measured third-order susceptibilities are compared to those measured in the optical region. At 1843 cm(-1) for CS2 the linear dipole polarizability correlation function adequately describes the nuclear contribution to the third order susceptibilities; for benzene and benzene derivatives, the linear polarizability dominates the nuclear response, but contributions from the dipolar and hyperpolarizability may need to be considered.