The surface tensions of binary salt mixtures made up from NaCl and MgCl2 in aqueous solution were measured as a function of concentration and temperature. By analyzing the results, the thermodynamic quantities of surface formation such as entropy (Delta s), Helmholtz free energy (Delta f), and energy of surface formation (Delta u) were evaluated. The Delta f of the mixtures positively deviates from the straight line connecting those of NaCl and MgCl2 aqueous solutions. On the other hand, Delta s shows negative deviation from the linear relation.