Four‐wave mixing experiments are used to study the variation of the third‐order susceptibility &khgr;(3), with frequency difference &Dgr;&ohgr; and laser intensityIin low carrier concentration HgCdTe crystals. At small &Dgr;&ohgr;, &khgr;(3)is caused by nonparabolicity of free electrons generated by two‐photon absorption, with &khgr;(3)scaling as &Dgr;&ohgr;−1andI2/3. The &Dgr;&ohgr; variation of &khgr;(3)indicates that the electron thermalization time is longer than 8 ps. At large &Dgr;&ohgr;, &khgr;(3)≃3×10−8esu is mainly due to bound electrons.