We measure the third-order nonlinear optical response of various dielectrics and semiconductors using the spectrally resolved two-beam coupling method at 2.3 µm, 3.5 µm, 4.5 µm, and 8.3 µm. These materials include fused silica, sapphire, calcium fluoride, magnesium fluoride, zinc sulphide, and zinc selenide. We compare our results with previous literature results and theoretically expected values using two-band model theory. The dispersion of the nonlinear refractive index n2 over this wavelength range is found to be negligible.