The zeta potential of a close-packed insoluble monolayer of 1-hexacosanol at the air-water interface in the presence of 0.001 M NH4NO3 was measured as a function of pH by means of the plane interface technique. The negative zeta potential increased with the increase of pH up to about 9. The negative charge at the monolayer-water interface is considered to be due to the dissociation of protons from hydrated alcohol water clusters at the surface. The zeta potential results were analyzed in terms of the site dissociation and site-binding mechanism within the Gouy-Chapman-Stern-Grahame double-layer model of the metal oxide-water interface.