The thermodynamics of electrified interfaces (electrodeelectrolyte and colloid systems) are re-examined, leading to a quantitative expression for the lowering of interfacial tension (surface pressure) due to the adsorbed ions in terms of hydrostatic and electrostatic contributions. Expressions are derived for the variation of these quantities across the double layer. It is shown that the classical expression for the double layer free energy leads to an incorrect result when applied to a double layer with a mobile layer of surface ions. Explicit results are given for a simplified model for the diffuse ionic layer based on a mean-field approximation (Gouy-Chapman theory).