Monodisperse CdS particles with mean diameter 0.51 mu m having 3.33% coefficient of variation (relative standard deviation) in size distribution were obtained in large quantities by instantaneous addition of 5 ml of 1.2 mol dm(-3) thioacetamide (TAA) containing 1 wt% gelatin to 20 ml of Cd-EDTA solution consisting of 0.300 mol dm(-3) Cd(OH)(2), 0.303 mol dm(-3) EDTA . 2Na, 2.0 mol dm(-3) CH3COONH4, 0.6 mol dm(-3) NH3, and 1 wt% gelatin, After the addition of the TAA solution, the system was aged for 8 h at 60 degrees C under agitation. The yield was 91.5%. Uniform ZnS particles of mean diameter 0.73 mu m with a 4.08% coefficient of variation in size distribution were also obtained by analogous preparation procedures in which Zn(NO3)(2) was used in place of Cd(OH)(2).