The reduction of AuCl4- to colloidal gold in the presence of aliphatic alcohols and sodium dodecyl sulfatein aqueous solutions using 20 kHz ultrasound is described. The extent of reduction of AuCl4-was foundto be directly dependent on the gas−solution interfacial activity of the solutes used. The diameters of thegold colloids produced were in the size range of 9−25 nm. The particle size was dependent on the alcoholconcentration in solution and on the hydrophobicity of the alcohol. For a particular alcohol concentration,smaller particles were obtained the greater the hydrophobicity of the alcohol. A detailed mechanism ispresented that describes the overall sonochemically initiated reduction of AuCl4- leading to the formationof colloidal gold.