Transient J-aggregation of 1, 1'-diethy1-2, 2'-cyanine chloride (Dye 1) from dilute aqueous solutions catalyzed by nascent ultrafine silver bromide particles has been studied by a stoppedflow multichannel spectrophotometry in which Dye 1 solutions containing potassium bromide are mixed with dilute silver perchlorate solutions in a rapid solution mixing device and the time-resolved absorption spectra of the mixtures are recorded at intervals of 10-100 ms. Decay of monomer absorption band of the dye occurred immediately after solution mixing, with concurrent growth of a transient J-band. The rate of this process was larger at lower temperatures and paralleled the rate of silver bromide nucleation, indicating that the latter was the rate-determining step. The reaction order of the catalyzed J-aggregation was 2-3 when it was determined from the dependence on dye concentration of the growth rate of the J-band, while it was a little larger than unity when determined from the decay rate of the monomer band. These suggest that the adsorption of Dye 1 molecules to silver bromide surface as monomers competes with the catalyzed J-aggregation. At higher silver bromide concentrations, monomer adsorption preceded J-aggregation, the latter being probably a surface process. The J-band under such conditions was broad and blue-shifted by about 10 nm.