Gold leaching by thiourea has been investigated in an alkaline solution. In the presence of both Na2SO3 and Na2S2O8, the chemical dissolution of gold can be accomplished in the alkaline thiourea solution. It was found that used as a buffer, SO32- can prevent thiourea from further decomposing into some insoluble species in contact with the oxidant S2O82- and that the mixed potential of gold is an important factor influencing thiourea decomposition and the rate of gold dissolution. Time dependence of the mixed potential and the dissolution mass for gold were also evaluated during the leaching process. This alkaline thiourea solution can be applied to gold extraction from the gold resources containing silver, copper, nickel, and iron, because these metals normally do not dissolve in the solution.