Using electrochemical reductive desorption of surface modifiers the structure of the modified surface of a single crystal electrode was studied. A very small amount (e.g. 1 mol %) of sulfide impurity in a 4-pyridinethiol (4-PySH) modifier solution was found to adsorb on an electrode in competing with 4-PySH and eventually adsorbed 4-PySH (or bis(4-pyridyl)disulfide, 4,4'-PySSPy) molecules were replaced completely by sulfide. In an ethanolic solution the sulfide impurity affected the modified surface structure of an Au(111) electrode more significantly than in an aqueous solution. The purified 4-PySH and 4,4'-PySSPy samples reproducibly gave a preferable surface for cytochrome c electrochemistry. The reductive desorption peak potential of a thiol adsorbed on the electrode surface was suggested to be a measure for predicting the structure of the modified surface. When the desorption peak potential of thiol becomes more positive, faster replacement of thiol by sulfide occurs. On the same lines, it was explained why an Au(111) single crystal surface is more sensitive to the sulfide impurity than are the Au(100) and Au(110) surfaces.