The adsorption isotherms of probe cationic molecules were measured at various electrolyte solution interfaces by resonant second harmonic generation. The excess charge density was obtained by analyzing the isotherms; it increases with square root of the bulk electrolyte concentration. Its value is ion-specific and the amount of probe molecular adsorption follows the Hofmeister series. By calculating the pressure anisotropy at the interface, it is found that the ratio of surface tension increment to the bulk electrolyte concentration decreases with the square of the excess charge density. This is in good agreement with the experimental observations.