AbstractUsing linear sweep polarography (LSP) in an acidic solution (pH 2.8) containing Au(III) ions andN‐allyl‐N′‐(sodium‐p‐phenylsufonate)thiourea (NTu), the product of the reaction between Au(III) and NTu (NDF) is reducible at the mercury electrode with two waves at about −0.40 and −0.95 V [versus the saturated calomel electrode (SCE)]. In the presence of excess chloride, the potential of the first wave shifts positively to −0.35 V. This forms the basis of a highly selective and sensitive method for gold. The linear range for gold is from 1 × 10−7to 3 × 10−M, and the detection limit is 5 × 10−8M (9 ppb). This method has been used for the determination of gold in ore samples, and the results agree well with values obtained by accepted procedures. Various electrochemical techniques were used to study the mechanism of the redox process. The reaction mainly involves the formation of C,C′‐di‐N‐allyl‐N′‐ (sodium‐p‐phenysulfonate)dithiodiformamidine (NDF). Unlike some explanations reported in the literature about the reduction of formamidine derivatives (containing an SS bond) on an Hg electuode, the electrode process for the first wave (−0.40 V) is ascribed to the reduction of the HgS film, which is caused by the reaction of Hg with adsorbed NDF. In the presence of excess Cl‐ ions, the mixed precipitation film of HgS·xHg2Cl2may be formed on the electrode surface and make the first wave shift to −0.35 V. The second wave