Decomposition of peroxypropionic acid (PPA) takes place in the liquid phase, but the main products of decomposition, carbon dioxide and oxygen are transferred to gas phase. An analytical method was developed to determine the decomposition of PPA in liquid phase by means of chemical analysis of gas phase. The method is based on on-line mass spectroscopy (MS). A mathematical model for the semi-batch gas–liquid system was developed. The model comprised both kinetic and mass transfer effects. A comparison between experimental results and results predicted from the mathematical model revealed that the model can describe the essential effects of decomposition kinetics.