Coke formation in C-5/C-6 alkane isomerization on a Pd/H-Mordenite catalyst discharged from a pilot reactor was studied. Experimental techniques like XRD, EPR, MAS-NMR, FTIR, TGA and TPO, were applied for the characterization of coke and coked catalysts. It was shown that coke content, chemical composition, its structure and nature depended upon time-on-stream and section of the catalyst bed. The coke content increased from 3.0 in the top section, to 4.1 wt.% in the central section and 6.2 wt.% in the bottom of the catalyst bed and C/H ratio of the corresponding coke increased from 1:1.12, 1:0.55 to 1:0.35, which significantly affected the catalytic behavior and regeneration of the catalysts. The selectivity to 2,2-dimethyl butane during n-hexane isomerization, respectively, decreased from 19% for the fresh catalysts, to 10% and 8.3% for the coked catalysts in the central and outlet sections. As time-on-stream increased, coke initially formed on palladium metals and then moved to acidic sites in the support where polyaromatic or pseudographite-like structures were formed through further acid catalyzed reactions. Two kinds of coke with different nature could be combusted below 300 and 600 degrees C, respectively. These results are very useful for choosing operation parameters to restore initial activity of the coked catalysts in the regeneration units in our pilot plant. (C) 2000 Elsevier Science B.V. All rights reserved.