We used the monolayer particle interaction apparatus to measure the force between a monolayer of stearic acid or octadecanol at the air/water interface and a colloidal silica sphere. The silica sphere approached the monolayer from the aqueous subphase. The aim was to analyze how the magnitude of the charge of a deformable interface affects the interaction between that interface and a like-charged hard particle. The charge density of the stearic acid monolayer was controlled by adjusting the pH (5.8-9) and the surface pressure. The octadecanol monolayer acted as a reference; the alcohol headgroup did not dissociate between pH 5.8-9.0. Stable monolayers of dissociated stearic acid molecules were formed at the air/water interface by dissolving stearic acid into the subphase to give a saturated concentration at each pH value studied. The approach force curve showed that the electrostatic repulsion increased with an increasing degree of dissociation and therefore the charge of the monolayer. The strength of the repulsion corresponded to that measured between two like-charged hard surfaces, but the apparent range of the repulsion was larger for a deformable interface. Retracting force curves displayed a significant adhesion, whose magnitude and range depended on the surface pressure and subphase pH.