The application examples of PTR-MS which measures volatile organics forming flavor or quality of foods in real-time were introduced focusing on breath-by-breath analysis of retronasal aroma. Time variations of multiple low concentration flavor compounds at each exhalation events after swallowing food or drink could be tracked simultaneously by using PTR-MS instruments. With this approach, the aroma perception was analyzed in view of mass transfer processes, especially remarking interfacial mass transfer which determined liquid–gas partitioning. Physicochemical properties related with retronasal aroma were complicated and could not be interpreted only by the differences of simple Henry’s law constants.