Surface tension data have been obtained by the method of differential capillary rise on the binary systems N2+ Ar and N2+ CO at 83.82°K, and on the systems N2+ CH4, Ar + CH4, and CO + CH4 at 90.67°K. Thermodynamic equations are presented for the prediction of the surface tensions of simple mixtures, in which the surface phase is considered to be a regular solution. Their use is illustrated for the five systems studied.