The relative surface potentials of Langmuir monolayer phases at the air/water interface have been determined in the three-phase coexistence region gas/liquid expanded/liquid condensed of methyloctadecanoate. The monolayer is assumed to be in hydrostatic equilibrium. A force balance applied to the morphology of the monolayer observed with fluorescence microscopy directly yields the relative surface potentials. This enables the determination of surface potential variations on a micron scale.