The Concentration-Controlled Recycle Reactor. The scope offered by the concentration-controlled recycle reactor for elucidating the kinetics of heterogeneous gas reactions is. presented in three contexts : identification of reaction networks; experimental design; and separate determination of changes of activity. Control strategies appropriate for the problem at hand are an essential prerequisite for the application of experimental strategies. In particular, long analysis cycles require appropriate modification of modern methods of control theory. Various model-based control concepts have been developed and tested by experiment and simulation.