Ultrafine PbS particles were prepared using AOT/isooctane reverse micellar systems as reaction media. On the basis of the change in UV-visible absorption spectra expressing the change in the size of semiconductor particles, the mechanism of the particle formation was studied. Because of the relatively large solubility of PbS, the growth of particles by Ostwald ripening was significant at low reactant concentration. When an excess amount of lead ion or sulfide ion was used, ultrafine PbS particles having stable spectra could be prepared, Particle coagulation was found to be controlled by the statistical distribution of particles among the reverse micelles. By irradiating a reverse micellar solution containing PbS particles stabilized by an excess amount of sulfide ion using a Xenon lamp, hydrogen was generated by the photocatalytic reduction of water.