A new technique for tracer-exchange experiments with positron emission profiling was developed which permits the spatially resolved, in situ investigation of kinetic processes in fixed-bed reactors under steady-state conditions. Modeling of the complete reactor proves to be a valuable tool for the design and interpretation of experiments. To validate the new method, the micropore diffusion of 2-methylpentane and n-hexane in silicalite was investigated. For n-hexane, a diffusivity of 7 x 10−8 cm2/s was measured at a temperature of 413K with an activation energy of 19kJ/mol. The diffusivity of 2-methylpentane at the same temperature was 1.0 x 10−8cm2/s with an activation energy of 29kJ/mol, clearly showing the influence of the additional steric hindrance due to branching. Tracer-Exchange Experiments with Positron Emission Profiling: Diffusion and Conduction in Zeolites. R.R.Schumacher, B.G.Anderson, N.J.Noordhoek, F.J.M.M.De Gauw, A.M.De Jong, M.J.A.De Voigt, R.A.Van Santen: Microporous and Mesoporous Materials, 2000, 35-36, 315-26