Differences in the extent of protonation of functional groups lying on either side of water–hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol–gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H2O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air–water interface predict that such event is hindered by a significant kinetic barrier unless OH− ions are present therein. Thus, by detecting RCOO− we demonstrate the presence of OH− on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me)3N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH− densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH gradients in the interfacial region. This fact could account for the weak OH− signals detected by surface-specific spectroscopies.