An exptl. set-up for the simultaneous detn. of reactant concn. in the gas phase and on the surface by spectroscopic methods was proposed to obtain information on the intrinsic kinetics of unsteady-state processes undergoing heterogeneous catalysis. The hydrogenation of CO and CO2 and the effect of the water-gas shift were discussed. Transient expts. were not always sufficient for model discrimination. Among models describing steady-state and transient behavior of the addn. of AcOH to C2H4 and of elimination reactions, model discrimination was possible only on the basis of forced concn. expts. [on SciFinder (R)]