The mechanism of formation of ultrafine CdS, ZnS, and their composite particles by the injection of H2S into reverse micelles was studied. The particle formation process was followed by the change in UV-visible absorption spectra. The kinetics of the whole process including dissolution of H2S; nucleation, particle growth, and coagulation was analyzed from time-course changes of the size and number of formed particles. The dissolution of H2S was the principal rate-determining step, and most of the dissolved H2S was consumed for particle growth. The particles formed in the present gas injection method were larger in size than those in the previous solution-mixing method in most cases. A kinetic scheme based on the distribution of the species among the micelles was then proposed, and this successfully explained the particle growth. Composite particles of CdS and ZnS having mixed crystal or core-shell structures were also prepared, and the application of these particles as photocatalysts was investigated.