The encapsulation of US nanoparticles, immobilized on thiol-modified polystyrene particles (PSt-SH), with polythiourethane (PTU) has been investigated. US nanoparticles, prepared in reverse micellar systems and immobilized on PSt-SH, were found previously to be unstable under visible light irradiation (J. Phys. Chem. B 2000, 104, 11639), but can now be stabilized effectively, against photoirradiation, by encapsulation within PTU, formed by using 1,2-ethanedithiol and m-xylylene diisocyanate as monomers. The resulting (PStCdS)PTU composite has been utilized for photocatalytic H-2 generation from 2-propanol aqueous solution, and it has been found that the wavelength region of photoabsorption can be controlled easily by changing the water content of the reverse micellar system, used for the US nanoparticle preparation, and encapsulation of PSt-CdS by PTU also retains the quantum size effect during photoirradiation.