Reactive distillation means the simultaneous implementation of reaction and distillation in a counter-current column. The effective phase equilibrium is very complex since the chemical equilibrium is superimposed on the vapor liquid equilibrium. The paper focuses on the calculation and presentation of this equilibrium using so called reactive distillation lines, This leads to a clear explanation for the existence of reactive azeotropes and to a simple method for the determination of the concentration of these azeotropes. The effectiveness of this method is demonstrated on several mixtures with different reaction mechanisms.