The formation mechanism of uniform AgCl nanoparticles in a reverse micelle (RM) system of polyoxyethylene (6) nonylphenyl ether/water/cyclohexane has been investigated through a new approach that uses a double jet technique, in which AgNO3 and KCl in RM solutions were continuously introduced at the same time. The final particle number was proportional to the feed rate of the reactants, in the same way as in the conventional double jet process in an aqueous gelatin solution (AGS) system. The experimental results in both systems were in excellent agreement with the theoretical prediction that was based on a diffusion-controlled growth model, where the reverse micelles that contained Ag ions in the RM system primarily carried only one Ag ion, and thus they were regarded as a type of Ag-ion complex that diffuses in the organic medium to be reacted with AgCl particles that have been stabilized by the surfactant. Thus, the dramatic reduction of particle size in the RM system was due to the extremely small growth rate of the nuclei, which was caused by the very low solubility of the solid in the RM solution, in terms of the concentration of the micellar Ag-ion complex in the organic medium and the small diffusivity of the bulky Ag-ion complex, in contrast to the conventional explanation, mainly in terms of the limited dimensions of the reverse micelles as nanotemplates. The exceedingly low substantial solubility of solid seems to be, at least, one of the main reasons for the formation of ultrafine nanoparticles in general RM systems.