CuxS nanocrystals (NC’s) are reported. The samples are prepared by a CdS‐to‐CuxS chemical conversion from the glasses originally containing CdS NC’s. A room‐temperature linear absorption of the converted samples shows several well resolved peaks with spectral positions from red to blue. These spectral features are explained by size quantization within CuxS NC’s (∼4 nm radius) with different copper deficiency [xis in the range from 1.8 (digenite) to 2 (chalcosite)]. A strong bleaching of the samples with a 3‐ns relaxation is observed in the pump–probe measurements. A high value of the third‐order nonlinear susceptibility (∼10−7esu) is derived from the nonlinear transmission spectra. ©1995 American Institute of Physics.