Electric charge at the water-water interface of demixed solutions of neutral polymer and polyelectrolyte decreases the already ultralow interfacial tension. This is demonstrated in experiments on aqueous mixtures of dextran (neutral) and nongelling fish gelatin (charged). Upon phase separation, electric charge and a potential difference develop spontaneously at the interface, decreasing the interfacial tension purely electrostatically in a way that can be accounted for quantitatively by Poisson-Boltzmann theory. Interfacial tension is a key property when it comes to manipulating the water-water interface, for instance to create novel water-in-water emulsions.