The magnitudes of the nonlinear optical susceptibilities of trigonal selenium, cadmium telluride, and indium antimonide have been measured at 28.0 &mgr;m relative to d11(tellurium). By comparison to the recently measured absolute value for tellurium, d11(Te)=(5.7±1.6×10−10m/V, the values obtained in this work are d11(Se)=(18.4±8.6)×10−10m/V, d14(CdTe)=(5.9±2.4)×10−11m/V, and d14(InSb)=(5.6±2.3)×10−10m/V. For Te, CdTe, and InSb, the measured 28.0 &mgr;m value is less than the 10.6 &mgr;m value, while for Se, the 28.0 &mgr;m value is considerably larger than the 10.6 value. These seemingly inconsistent results are explained using Garrett's anharmonic oscillator model.