The addition of Zn to H-ZSM-5 zeolites was studied for application to endothermic reforming in hypersonic aircraft engines. Temperature-programmed-desorption (TPD)/thermogravimetric-analysis (TGA) measurements with 2-propanamine on two H(Zn)-ZSM-5 samples showed that at low ion-exchange levels, less than 0.5 Zn/Al, each Zn cation displaces one Bronsted-acid site. Although rates for n-hexane conversion at 633 and 823 K and at a pressure of 137 bar decreased with the loss of Bronsted sites, Zn promotion greatly increased the production of H-2 and the formation of small aromatic molecules. FTIR of adsorbed acetonitrile-d(3) and calorimetric measurements of adsorbed CO at 195 K indicate that the exchanged Zn cations form Lewis-acid centers. A model in which the Zn cations, acting as Lewis-acid centers, polarize intermediates formed at Bronsted sites is presented as a way of understanding the observations.