We have studied the pH-dependent speciation of aqueous HAuCl 4 and its influences on the synthesis, structure, and property of Au colloids. Aqueous HAuCl 4 consists of [AuCl x (OH) 4− x ] − ( x ≥ 2) at low pH but [AuCl x (OH) 4− x ] − ( x < 2) at high pH. By employment of ascorbic acid as the red…