The voltammetric response of a Ag(111) electrode coated with a mixed molecular film of 11-mercaptoundecanoic acid and 1-decanethiol is presented. The voltammetric current arises from the reversible, electric field-driven deprotonation of carboxylic acid groups. Voltammetric peak heights for interfacial protonation and deprotonation are a function of solution pH, obtaining maximum values near the pK(1/2) (similar to 8.5) of the molecular film.