The voltammetric response of a Ag(111) electrode coated witha mixed molecular film of 11-mercaptoundecanoic acid and 1-decanethiol is presented. The voltammetriccurrent arises from the reversible, electricfield-driven deprotonation of carboxylic acid groups. Voltammetricpeak heights for interfacial protonationand deprotonation are a function of solution pH, obtaining maximumvalues near the pK1/2 (∼8.5) ofthemolecular film.