Mathematical models of surface tension as a function of solute concentration are needed for predicting the behavior of surface processes relevant to the environment, biology and industry. Current aqueous surface tension-concentration models capture either solutions of electrolytes or those of organics, but a single set of equations has not yet been found that represents both in one unified framework. In prior work we developed an accurate model of the activityconcentration relationship over the full range of compositions by extending the BET and GAB isotherms models to multiple sorbed monolayers (Dutcher et al. JPC 2011, 2012, 2013). Here we employ similar statistical mechanical tools to develop a simple equation for the surface tension-composition relationship that differs remarkably from prior formulations in that it (1) works equally well for organic and electrolyte solutes and their mixtures, (2) does not contain any factors representing the relative amounts of solute in the bulk or at the surface-this is captured by surface-bulk equilibria in the model, and (3) is accurate over the entire RH range.