The oxidation of monolayers of 1-octadecanethiol to dioctadecyl disulfide over a subphase of potassium hexacyanoferrate(III) was investigated. The reaction was accelerated by the presence in the monolayer of even a minute amount of octadecylamine, which acted as a catalyst. As the amount of amine in the monolayer was increased, the rate of the reaction increased. A stage was reached at which a further increase in the catalyst concentration did not cause a commensurate increase in the rate. This has been interpreted as indicating a clumping of amine molecules in the monolayers at higher concentrations. The reaction is also catalyzed by OH- contained in the subphase. Over both acidic and basic solutions, however, the effectiveness of the amine as a catalyst is reduced compared to its effectiveness over the neutral subphase.