Pennyroyal essential oil was isolated by supercritical fluid extraction and fractional separation. Extractions were performed at three different mean particle sizes (0.3, 0.5 and 0.7 mm) and three CO2 flow rates (0.31, 0.43, and 0.62 g/s) and at p=100 bar and T=323 K. Essential oil yield was determined as a function of time. Yield data and physical considerations based on the botanical structure of pennyroyal leaves were used to screen the possible mass transfer mechanisms. Two mathematical models were constructed, based on the numerical integration of differential mass balances written along the extraction bed. They take into account the desorption of essential oil located near the leaf surface and the mass transfer resistance to the extraction of the part of essential oil contained in the internal part of the vegetable structure. Axial dispersion was also taken into account. Yield curves for all particle sizes and CO2 flow rates were fairly well fitted using the internal mass transfer coefficient Ki as the only adjustable parameter of the model. The best fit value was Ki=1.4×10−7 m/s. Once the model was validated on the experimental data, simulations were performed for (a) different partitions of essential oil between the surface and internal cells of leaves, and (b) a larger range of CO2 flow rates. Simulation results can be applied to other vegetable species and to determine the performance of this process on a larger scale plant.