Electrochemical methods were used to study the effect of ammonia on the anodic process of gold dissolution in a copper-free thiosulfate solution. Ammonia changes the thiosulfate oxidation behavior on gold by weak adsorption. Raman spectra confirmed that ammonia is able to reduce the passivation film which contains polysulfides. The mechanism of gold dissolution in the solution mixture of thiosulfate and ammonia is similar to that in the solution containing only thiosulfate. Ammonia promotes gold dissolution because it prevents the formation of intermediate products during the thiosulfate oxidation process. That's also why it can reduce the partial passivation of the gold surface. (C) 2016 The Electrochemical Society. All rights reserved.