The formation of colloidal gold was kinetically studied using the monomer or micelle form of the acetylenic glycol nonionic surfactant Surfynol 465, which functions as a reducing and stabilizing agent. When two equal volumes of aqueous solution of Au(III) and Surfynol 465 were mixed, absorbance at the peak around 300 nm due to Au(III) rapidly decreased with time exponentially. A new peak began to appear around 530 nm, whose height sigmoidally increased with time, showing the formation of colloidal gold. From the variation in absorbance at 300 or 530 nm with time, colloidal gold formation would appear to occur through consecutive firstorder reactions, Au(III) 3 Au(I) 3 Au(0) 3 gold particle, for monomer and micelle systems. As for Au(I), the complex between Au(I) and OC'CO of Surfynol 465 was assigned based on reactions previously proposed for the Raman spectra of colloidal gold (Sato, S., Colloid Polym. Sci. 274, 1161Sci. 274, (1996))). The reaction, Au(0) 3 gold particle, proceeded quickly. The overall rate of reaction was slower in the micelle system than in the monomer system. For both systems, the rate constant of the reaction Au(III) 3 Au(I) exceeded the constant of the reaction Au(I) 3 Au(0), and the activation energies of the two reactions were 58 kJ mol Ψ1 . From the agreement with the activation energy of formation of colloidal gold by another method, consecutive first-order reactions, Au(III) 3 Au(I) 3 Au(0) 3 gold particle, were considered valid for the formation of colloidal gold in the presence of Surfynol 465.