Step-up (adsorption) and step-down (desorption) transient experiments were both simulated and performed in a recycle reactor in order to determine simultaneously diffusion coefficients and reaction rate constants for the multicomponent system ortho-, meta-, para-xylene on ZSM-5 type zeolites. Therefore an unsteady-state simulation program based on a continuum approach was developed for studying the expected phenomena and for predicting optimum catalyst data and operating conditions due to a maximum influence:of the diffusion/reaction interaction on the system response. In order to check the accuracy of the diffusion coefficients determined by this method step-up experiments on a nonactive silicalite were performed and analysed. For a temperature of 210 degrees C and partial pressures lower than 100 mbar the diffusion coefficient of ortho-xylene was determined to 5.0 +/-3.0 x 10(-17) m(2)/s in the single-component system and 3.5 +/-1.5 x 10(-16) m(2)/s in the multicomponent system. The ratio RD of the diffusion coefficients of para- to ortho-isomer was within the range of 30 +/- 10. This is consistent with the literature.