The optical absorption of ZnS semiconductor nanocrystals inside pores of silica has been investigated. These ZnS nanoparticles were synthesized through sol–gel processing, followed by firing. X-ray diffraction and nitrogen adsorption/desorption results show that the ZnS nanocrystals are highly defective. Based on the analysis of x-ray and nitrogen adsorption–desorption results, the surface layers of ZnS nanocrystals can be regarded as being amorphous. It was found that, compared with that of the bulk ZnS, the optical absorption edge of ZnS nanoparticles shifts to a longer wavelength. We believe that the amorphous surface layers of ZnS nanocrystals can mainly be responsible for the observed redshift of the optical absorption edge. ©1997 American Institute of Physics.