Thiol-passivated AgI nanoparticles of 7-15 nm were synthesized in large quantities by changing the initial molar ratio of iodide to 1-thioglycerol, and thus their competitive reaction rates with silver ions was adjusted, leading to a precise control of the final particle size. Powder X-ray diffraction and transmission electron microscopy showed that these particles crystallized as a mixture of beta- and gamma-AgI, with the content of the gamma-phase increasing from 26% for 7 nm to 60% for 15 nm. Elemental analysis, thermogravimetric analysis, and Fourier transform infrared spectroscopy indicated that thiolate anions are attached to the AgI surface through mercapto groups, and furthermore, the silver thiolate molecular complex exists as a result of the competitive reactions. The percentage of the initial thiolate used for capping AgI particles increased linearly with the increase in [I-]/[1-thioglycerol], showing that almost pure thiol-covered AgI particles of 15 nm can be obtained. Optical spectroscopic studies in diluted solution confirmed the above competitive mechanism of AgI formation. The smallest AgI particles stabilized by 1-thioglycerol and surviving several days in water, observed in a solution of 3.33 x 10(-4) M Ag+, 6.67 x 10(-4) M I-, and 6.67 x 10(-3) M 1-thioglycerol at room temperature, had an absorption peak at 331 nm.