An SECM method for measuring lateral proton diffusion at Langmuir monolayers deposited at a water/air (W/A) interface is described. The approach utilises the electrogeneration of a base by the reduction of weak acid (producing hydrogen) at an inverted 'submarine' ultramicroelectrode (UME) placed close to the monolayer in the aqueous subphase of a Langmuir trough. The electrogenerated base diffuses to the monolayer and titrates away protons, resulting in conversion of the base back to its initial form. The acidic monolayer is thus depronated locally which creates a concentration gradient for lateral proton diffusion. A numerical model is developed for the SECM mass transport problem and the effect of lateral proton diffusion on the UME current response is established. The method is demonstrated experimentally with measurements of lateral proton diffusion along a stearic acid monolayer at a wide range of surface pressures. The lateral proton diffusion coefficient is found to be fairly uniform (∼8 × 10-6 cm2 s-1) when the surface pressure of the monolayer (π) is in the range of 1-20 mN m-1, but decreases to immeasurably low values when π is greater than 25 mN m-1.