Hypothesis: The origin of negative surface charge at water/air interface is still not clear. The most probable origin is specific adsorption of OH- ions. From diffuse layer potential, we can evaluate the surface density of ions in the Stern layer which can be a measure for the specific adsorption of ions and determines whether the surface charge is solely due to the specific adsorption of OH- ions. Experiments: Equilibrium thickness of foam films of pure water and aqueous solutions of NaCl, HCl, and NaOH was measured as a function of disjoining pressure for water and as a function of concentration for the aqueous solutions at 298.15 M. Quartz-glass cells thoroughly cleaned and immersed in pure water before use were used for the measurement. Findings: Application of a modified Poisson-Boltzmann equation to the equilibrium film thickness gave the diffuse layer potential and the surface density of ions in the Stern layer. From the concentration dependence of the surface density, it was concluded that not only OH- ions but also Cl- ions and HCO3- and/or CO32- ions adsorb specifically at the water/air interface. (C) 2019 Elsevier Inc. All rights reserved.