Zinc sulphide nanoparticles confined in amorphous and porous silica thin films were prepared by using sol-gel (spin coating) method on quartz (fused silica) substrates. Optical behaviour of the nanoparticles in partially densified silica thin film hosts was studied as a function of annealing temperature (473-573 K) for a fixed annealing time (30 min). The transmission spectra were utilized to estimate the band gap without pre-assuming the nature of optical transition in the film. The band gap varied between 4.27 and 3.96 eV as a function of increasing annealing temperature. The average radii of the nanoparticles, determined from transmission electron micrographs, varied between similar to2 and 4 nm which matched well with that obtained from blue shift of the band gap. Electron diffraction patterns showed distinct rings due to (10 0), (0 0 2) and (10 2) planes. The Urbach tail widths, estimated from the absorption spectra, were found to decrease with increasing particle size. The refractive indices of the composite films varied in the range 1.8-2.2. (C) 2002 Elsevier Science B.V. All rights reserved.