We propose anew method for phase equilibrium measurements where zero net mass flux over the phase boundary is obtained even at chemical non-equilibrium. Thus, physical equilibrium can be guaranteed irrespective of (potentially slow) mass transfer. We have used this method for the experimental determination of vapor-liquid equilibria (VLE) for a reactive transesterification system in a new type of flow-through still where compositions of both phases are observed in situ by infrared spectroscopy to avoid disturbance of the steady state. The design circumvents limitations of conventional apparatus and allows measurements for promising but demanding applications like reactive distillation. Besides, the data obtained with this method can be used to develop kinetic information.