Ion specificity plays a key role in solution chemistry and many biological processes. However, the classical DLVO theory has not been able to explain the experimentally observed ion specific forces acting between air-bubbles in electrolyte solutions. We resolve this problem by using a generalized Poisson Boltzmann equation. We demonstrate that inclusion of both short-range potentials obtained from simulation (acting between ions and the air-water interface) and the spatial variation of the local dielectric constant near the air-water interface may be essential to obtain correct results. (c) 2008 Elsevier B. V. All rights reserved.