A semibatch reactor concept was proposed for the determination of the kinetics in complex catalytic liquid and gas-liquid systems with reactions that have highly varying rates. The method is based on continuous removal of the liquid phase from the reactor, while the catalyst remains inside the reactor. In this way, the catalyst bulk density (mass of catalyst, relative to the liquid volume) continuously increases, which enhances the secondary and tertiary reactions in the system. It becomes possible to determine all of the kinetic parameters from a single experiment. Mathematical models were presented and then solved analytically (first-order reaction systems) and numerically (general kinetics). The concept works in practice, which was demonstrated by an experimental study: the catalytic hydrogenation of citral on a nickel catalyst. The primary product (citronellal) is formed very rapidly, whereas the secondary (citronellol) and tertiary (3,7-dimethyloctanol) products appear much more slowly. A standard isothermal, constant-volume experiment in a slurry reactor cannot provide data from which all of the rate constants could be determined in a reliable way. With the proposed semibatch concept, the formation of the ultimate products was accelerated considerably, and all of the rate parameters were successfully estimated by nonlinear regression analysis. The proposed approach is not limited to semibatch slurry reactors; however, it can be extended to fixed beds with recycling, as demonstrated by computer simulations.