A commercial speed of sound (SOS) immersion probe has been modified in order to permit concentration measurements of liquid mixtures in the presence of dispersed gas phases. Experiments were performed in which water was removed from a polymer/water mixture via evaporation and stripping in mixed vessels of different flow patterns. The original and modified SOS immersion probe was used to monitor the water concentration. In the stripping stage, where a persistent gas phase of high volume fraction is generated, the original SOS immersion probe fails to measure the water concentration. This drawback has been successfully eliminated by means of a special SOS probe housing. The modified SOS immersion probe showed reliable inline water concentration monitoring for directed (draft tube reactor) and undirected flow pattern (ordinary stirred reactor) under boiling and stripping conditions.