Surface diffusion of adsorbed alkanethiolate molecules forming a binary self-assembled monolayer (SAM) on Au(111) has been studied by monitoring the change in the shape of voltammograms for the reductive desorption of adsorbed thiolates. Artificially phase-separated SAMs of undecanethiol (UDT) and mercaptoundecanoic acid (MUA), which would form homogeneously mixed binary SAMs at the thermodynamic adsorption equilibrium, are prepared starting from the phase-separated binary SAMs composed of mercaptopropionic acid (MPA) and UDT, followed by the electrochemical partial desorption of the MPA domains and successive adsorption of MUA from ethanol to the domains originally occupied by MPA molecules. The mutual dissolution of UDT and MUA on the gold surface is monitored as the gradual merging of the initially well-separated two peaks on the voltammogram of reductive desorption of the thiolates. The average diffusion coefficient of adsorbed UDT and MUA at 60 degreesC in water is estimated to be about 10-18 cm(2) s(-1).