The reduction of AuCl4- to colloidal gold in the presence of aliphatic alcohols and sodium dodecyl sulfate in aqueous solutions using 20 kHz ultrasound is described. The extent of reduction of AuCl4- was found to be directly dependent on the gas-solution interfacial activity of the solutes used. The diameters of the gold colloids produced were in the size range of 9-25 nm. The particle size was dependent on the alcohol concentration 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 is presented that describes the overall sonochemically initiated reduction of AuCl4- leading to the formation of colloidal gold.