A method for immobilization of metal sulfide ultrafine particles formed in reverse micellar systems in polythiourethane via bonding of the thiol groups to a metal sulfide surface is investigated. CdS or CdS-ZnS coprecipitated ultrafine particles-polythiourethane composites (CdS-PTU and CdZnS-PTU) were prepared by polymerization (polyaddition) of xylene-alpha,alpha＇-dithiols and hexamethylene diisocyanate in the presence of the metal sulfide ultrafine particles formed in a reverse micellar system and then surface-modified with thiophenol, The immobilized metal sulfide particles in the PTU composites are able to be characterized spectrophotometrically by measuring the scattered light from dispersions of the composites and found to show a quantum size effect. The particle exposed fraction, F, is defined as the ratio of the quantity of Cd or Zn dissolved from the composites by HCl to the quantity of the metal dissolved by concentrated H2SO4 (total analysis). The value of F can be controlled by the quantities of the monomers, and also by the value of W-0, (=[H2O]/[AOT]) of the original reverse micellar system, The PTU composites were applied to photocatalytic H-2 generation from aqueous solution of 2- propanol, They showed improved stability against photodegradation compared with the thiophenol-modified CdS particles before immobilization. The value of F seems to show the fraction of the metal sulfide particles effective for the photocatalytic reaction, and the photocatalytic activities of the PTU composites are possibly evaluated by using the quantity of H-2 generated per unit quantity of the exposed metal sulfide particles.