Coke formation and the effect of coke on transport and adsorption behaviour of a commercial HZSM-5 zeolite has been studied by means of an oscillating microbalance. This transient technique provides a powerful tool for in situ studies of adsorption and diffusion in coked zeolites. Adsorption experiments at 30 degrees C involving ethane, toluene and n-hexane were carried out before and after coke formation during ethene oligomerization at 475 degrees C using a partial pressure of ethene of 0.8 bar. Both adsorption rates and equilibrium adsorption were studied. Information about the location of coke was obtained from changes in characteristic diffusion time and adsorption capacity with coke content. Distinct deactivation modes such as site coverage, pore blocking and external coking were identified at different coke levels. Changes in characteristic diffusion time were found to depend on both coking level and coke location, which influenced either the diffusivity or the diffusion path.