Prediction of azeotropic properties of ternary systems from data on binary systems is of theoretical and practical interest. In this connection systems in which only the relations between the vapor pressures (boiling points) of the components and binary azeotropes require the existence of a ternary azeotrope merit detailed examination. The purpose of this paper is to examine certain characteristics of pressure surfaces of the saddle type in ternary systems with one binary azeotrope and to report an experimental study of liquid-vapor equilibrium in a system of this type. The system toluene-dioxane-isobutyl alcohol was chosen for the experimental investigation. A saddle azeotrope was found in this system by the ebulliometric method. Liquid-vapor equilibrium was studied at 80 and 100 degree C in a modified version of the Bushmakin apparatus. Isotherm-isobars for the solution at 80 degree are tabulated. The liquid-vapor equilibrium data are presented together with the results of calculation of equilibrium compositions in ternary systems from data on binary systems. The calculations were carried out by the method described by Susarev and Gorbunov for systems in which one of the binary systems is nearly ideal. Experimental data are discussed in terms of possible mechanism of phase equilbria.