A novel apparatus and method for time-domain tracking of high-speed chemical reactions. The apparatus of the present invention includes an RF heating system for heating a reaction zone and a probe system in the reaction zone for measuring the complex permittivity in the reaction volume. A feedback system controls the RF source by adjusting the frequency of the RF source as a function of the relaxation frequency as determined by the permittivity measured by the probe system. Advantageously, the novel apparatus and method of this invention is particularly useful for RF dielectric heating to recover products from oil shales since it was found that the optimum RF frequency for heating oil shale changes rapidly as the kerogen is heated to elevated temperatures.