The formation mechanism of monodisperse spherical CdS particles in a condensed homogeneous solution of Cd-EDTA complexes in the presence of thioacetamide (TAA), ammonium acetate, ammonia, and gelatin was studied. The monodisperse particles were formed through a consecutive process with an almost instantaneous nucleation and succeeding gradual growth without renucleation, The Cd-EDTA and TAA worked only as reservoirs of Cd2+ and S2- ions, but the rate-determining step of particle growth was found to be the dissociation process of the Cd-EDTA complexes and not of the TAA, Ammonia played a decisive role as a complexing agent at high concentrations of Cd(NH3)(n)(2+), which could promptly release free Cd2+ ions, in promoting nucleation as well as growth even under a low supersaturation, and thus achieved excellent monodispersity by lowering the supersaturation below the critical level during their growth. The important role of CH3COONH4-NH3 as a powerful pH buffer for maintaining the activity of both NH3 and TAA was also proved. Moreover, size control of the final product was achieved in a range from 0.038 to 0.51 mu m.