The hardness of the soft gold electrodeposited from the thiosulfate-sulfite mixed ligand bath which we developed recently, has previously been found to increase with increasing sulfur content of the gold deposit. In the present study the mechanism of the sulfur inclusion was investigated by analyzing for sulfur gold deposits produced under various experimental conditions and also by observing weight gains resulting from the adsorption of sulfur-containing species on gold using the quartz crystal microbalance (QCM) technique. The results led us to conclude that the origin of the included sulfur is primarily the adsorbed Au(I)-thiosulfate species, (Au2S2O3)(ads), formed as an intermediate in the gold deposition reaction.