Proton in-plane lateral diffusion rates in Langmuir monolayers have been measured as a function of surface coverage, with a sensitivity not possible previously, using scanning electrochemical microscopy (SECM) operating in the induced desorption mode. With this approach an acidic monolayer, spread at the air/water interface, is deprotonated locally, by reducing protons in solution to hydrogen at an ultramicroelectrode probe, which drives the acid dissociation reaction. In turn, this creates a proton diffusion gradient in the solution and at the interface, and the transport-limited current flowing at the electrode provides a measure of the rates of diffusion in these two environments. Measurements on stearic acid monolayers at the air/water interface clearly show that in-plane lateral proton diffusion occurs, but the diffusion coefficient depends critically on the physical state of the monolayer, and is at most only ca. 15% of the magnitude in bulk solution.