We report an experimental study of the nonlinear optical properties of HgTe-CdTe superlattices grown by MBE and Pb1−x,SnxSe grown by hot wall epitaxy. Nondegenerate four-wave mixing has been employed to measure third-order nonlinear susceptibilities at 10.6 µm as a function of temperature, laser intensity, and difference frequency. The nonlinearity is believed to be due to modulation of the free carrier temperature and density by the optical beams. The HgTe-CdTe results are compared to theoretical calculations based on free carrier contributions to the susceptibility, and the agreement with experiment is quite good. In 1-µm-thick Pb1−xSnxSe layers, multiple internal reflections of the light within the sample is found to enhance the nonlinear signal by an order of magnitude.