The oxidation of ferrocene on a defect-free mixed per-6-thio-beta-cyclodextrin/pentanethiol monolayer in a NaClO4 solution was inhibited in the presence of sulfate. The peak current in differential pulse voltammograms linearly decreased with the logarithm of the sulfate concentration in an extremely wide range from 1 nM to 1 mM, showing the possible application as an electrochemical sensor of sulfate.