This paper reports the effect of Er(2)O(3) concentration on nucleation and optical properties of PbS quantum dots (QDs) formed in a silicate glass under heat treatment at 490 degrees or 500 degrees C. As Er(2)O(3) concentration increased from 0.1 to 0.4 mol%, average radii of PbS QDs decreased from similar to 5 to similar to 4 nm, and peak wavelengths of absorption and photoluminescence also decreased. Clusters of Er3+ ions connected by bridging oxygens may act as the nucleation centers for PbS QDs.