We consider the distribution of ions of an unsymmetrical electrolyte of 2-1 type near an air/water interface and calculate the excess surface tension. For an unsymmetrical electrolyte, volume elements near the interface are no longer electrically neutral. However, overall electrical neutrality must be maintained so that there is a double diffuse charged layer with the charge density changing sign at a certain distance from the interface. The ionic concentrations thus depend on a mean potential ϕ as well as on the image-self-atmosphere potentials. By adopting the approximation of Oka, Onsager and Samaras of replacing the local Debye-Hückel constant by its bulk value, we derive a differential equation for ϕ which can be solved by variation of parameters. The excess surface tension is then obtained using a stress integral expression. Numerical results are computed for concentrations of 0.005, 0.01, 0.05 and 0.1 mol l.–1 and reasonable agreement found with the corrected experimental results of Jones and Ray for BaCl2.