The reaction mechanism of gas-phase ethylbenzene isomerization over bifunctional alumina- or zeolite-supported 0.5% by wt platinum catalysts was studied in a fixed-bed pulse microreactor by determining the product distributions in the conversion of ethylbenzene and of eight probable C/sub 8/ intermediates at 300/sup 0/-400/sup 0/C and 1.8 atm. Over the Pt/Al/sub 2/O/sub 3/ catalyst, xylenes are formed from ethylbenzene mainly via ethylcyclohexene to 1,2-methylethylcyclopentene to 1,2-dimethylcyclohexene to o-xylene, which corresponds to predictions from a bifunctional mechanism, in which only skeletal rearrangements of tertiary carbocations are considered to be rate-determining. Over Pt/zeolite catalysts with higher support acidity, skeletal isomerizations via both tertiary and secondary carbocations contribute to the measured product distributions. In ethylbenzene isomerization, the yield of C/sub 8/ naphthenes decreased from 60 mole % at 300/sup 0/C to zero at 400/sup 0/C and those of C/sub 8/ aromatics and xylenes passed through maxima of approx. 90% at 380/sup 0/C and 20% at 340/sup 0/C, respectively.