For the last 100 years, cyanidation has been the most important process in the extraction of gold from its ores. However, despite the large number of papers published on the reaction between gold and cyanide, there is little agreement about the kinetics and mechanism of this important reaction. In the present paper, the leaching reaction was studied as a function of solution purity using a rotating electrochemical quartz crystal microbalance. Surprisingly, the experimental results show that pure gold does not leach in highly pure aerated cyanide solutions. On the other hand, gold leaches at a measurable rate in solutions prepared using AR grade reagents. Therefore, it is not surprising that many of the past investigations have resulted in conflicting data. A model based on the formation and dissolution of a chain-like film of AuCN has been proposed to account for these results. It is believed that the dissolution of AuCN only occurs at the chain ends, and the presence of impurities in the cyanide solution accelerates this process by depositing on the gold surface and increasing the number of chain ends.