The differential variation of the method of capillary rise has been employed to measure the capillary constant and calculate the surface tension of ethane-nitrogen solutions. Experiments have been conducted in the temperature range from 93.15 K to 283.15 K at pressures up to 4 MPa. The (p, T)-projection of the line of three-phase liquid-liquid-vapor equilibrium and the surface tension at a liquid-vapor interface close to this line have been determined. Equations are developed which describe the dependence of the capillary constant and the surface tension on the temperature, pressure, and composition of the liquid phase. Experimental data on the surface tension are analyzed in the framework of thermodynamic models. (C) 2012 Elsevier B.V. All rights reserved.