Narrowly dispersed Pb-doped zinc sulfide nanoparticles were synthesized at room temperature using a chemical method in which the nanoparticle surfaces were passivated using mercaptoethanol. The maximum intensity of the broad green luminescence (similar to 530 nm) from these nanoparticles was observed at an optimum dopant concentration of 0.104 Pb wt%. The emission intensity was found to depend on the synthesis pH conditions, thus yielding maximum intensity at 5.0 pH. Comparatively lower emission intensities were observed for the other pH values (2.5-9.0 pH range). This may be due to the pH-dependent Cl- (as well as Na+) incorporation into the ZnS matrix, which possibly helps in inducing required density of impurity (donor/co-activator) states in the energy gap of ZnS nanoparticles. X-ray diffraction analysis using Debye functional analysis showed that the particle size is 2.8 +/- 0.3 nm. (c) 2005 Elsevier Inc. All rights reserved.