Hydrolysis of the propionic anhydrite catalysed with sulphuric acid at batch and semibatch operating conditions has been investigated using the reaction calorimeter RC1 Mettler Toledo. Due to a limited solubility of the anhydrite in the aqueous phase where the reaction takes place, mass transfer with simultaneous chemical reaction has to be considered. Contributions of both phases to the reaction mixture change during the reaction progress, so that a complex, strongly non-linear behaviour of the reactor has been noticed. Influence of the concentration of the catalyst, the reaction temperature as well as the initial volume fraction of the organic phase in the reaction mixture and the stirrer speed on the overall conversion rate have been determined directly from calorimetric measurements. Some indications related to the safe and efficient performance of the investigated process as well as to a simplified experimental kinetic model have been formulated.