In situ imaging technology developed for nuclear medicine is now being applied to study the kinetics of heterogeneous catalytic reactions under actual process conditions. Minute quantities of molecules (ca. 10-(15) mel), radio-labeled with positron-emitting isotopes such as C-11;N-13; or O-15, are injected as pulses into the feed streams of chemical reactors. Subsequent coincident detection of pairs of gamma photons produced via positron-electron annihilation allows the concentration of reactants, intermediates and products to be mapped as a function of both time and position within the reactor bed. As well as providing qualitative information regarding the mechanism of the reaction under investigation the data obtained can be compared with mathematical models based on the reaction kinetics in order to refine parameters such as activation energies and pre-exponential factors for elementary reaction steps. Since the technique is capable of imaging transient phenomena, information is provided that is not accessible to steady-state techniques.