Experiments to measure the effect of applied electric fields on the surface tension of aqueous solutions are difficult and have so far proved inconclusive. The effect of fields on both the surface pressure and surface excess of adsorbed solutes can be calculated from the simpler measurements of surface potential, using Maxwell relations from a thermodynamic analysis of the Volta effect. Examples for the air/water interface of NaCl and sodium dodecyl sulfate solutions are even. The results are compared with data for insoluble spread monolayers and contrasted with electrocapillarity. The effect of electric fields on the surface tension of water itself remains obscure.