The effect of reactants on coke formation during dehydrogenation (DH) over 0.3 wt.%Pt-0.3 wt.%Sn-0.6 wt%K/Al2O3 was investigated. The reactions were carried out in the temperature range 200-600 degrees C at 1 atm with pairs of alkanes and alkenes (C-3, C-5-C-8). The carbonaceous deposits formed during the process were analyzed using the temperature-programmed oxidation (TPO) technique. With the same amount of coke, the TPO profiles of the short alkane and alkene (C-3) did not match completely, while those of the larger chain alkanes and alkenes (C-5-C-8) were virtually identical. From these results, a simple model of coke formation has been proposed. Short alkane DH provides coke in series and larger alkane DH generates coke in series and consecutive modes. (C) 2000 Elsevier Science S.A. All rights reserved.