The first observations of three non-linear optical effects near the band gap in InSb at 4K and 77 K have been made, at very low light intensities, using a c.w. CO laser with a novel attenuator system. Non-linear refraction and absorption are found to increase strongly with intensity above 30 W/cm2 and, in addition at 4 K, the absorption decreases markedly for intensities from 1 mW/cm2 to 30 W/cm2 and thereafter increases. Possible origins of these effects are discussed. © 1978.