The objective of this study was the application of a high-performance compact reactor (HCR) in the treatment by activated sludge of a load polluted by hydrocarbons. The tests were carried out in the laboratory with synthetic water containing fuel oil as the principal source of carbon. A solution riched in nutrients was added continuously. At a first stage, the effectiveness of the process and its capacity to support high loads were highlighted. We regard the reactor as a perfectly agitated continuous reactor (PACR) with recycling of sludges. A simple mathematical model, based on the kinetics of the microbial growth of Monod and also on the kinetics of Michaelis-Menten for the use of the substrate, was used in order to evaluate the constant kinetics. The values of the kinetic parameters resulting from this study were compared with the data of the literature. The performance of the HCR system was confirmed by the high values of the specific speed of use of the substrate. The influence of the residence time of the cells on the quality of the effluent and the characteristics of sludges was discussed, and its importance as the control parameter of the process was examined. The HCR is distinguished from conventional systems with activated sludge by its compactness and its aptitude to treat significant organic loads, with very short hydraulic times of retention to produce an effluent of quality and activated sludge of good characteristics of decantability.