Although direct phase equilibria measurements of reacting systems are difficult to obtain generally, the thermodynamic behavior of a reacting system Such as that found in a reactive distillation processes can be constructed indirectly from measurements involving the phase equilibria of nonreacting pairs of compounds involved in the reaction in combination with an activity coefficient model. The binary data are used to regress binary interaction parameters for the chosen activity coefficient model. The ternary phase equilibria are estimated from the model. This work will describe the application of this method to the reactive system water/3-pentenoic acid/methyl-3-pentenoate. Vapor-liquid and liquid-liquid equilibrium measurements for the binary components were used to fit NRTL and UNIQUAC parameters. These activity coefficient models were then used to show how the composition-phase behavior can be constructed for the ternary system from the binary information only.