The kinetic isotope effect and the lack of deuterium-hydrogen exchange in the unconverted reactants show that chemisorption of the reactant is the slow step of the dehydrocyclization mechanism with a Pt-SiO2 catalyst. While the chemisorption was nearly irreversible for a Pt-SiO2 catalyst, this was not the case for a Pt-Sn-SiO2 and Pt-Mg(Al)O catalysts. Data from the competitive conversion of a mixture of n-C8H18 and n-C8D18 show that there is a kinetic isotope effect for the dehydrocyclization reaction and that the magnitude of the effect for Pt-Mg(Al)O (similar to 2.0) falls between those of Pt-SiO2 and Pt-Sn-SiO2. The slight reversibility of the chemisorption step indicates that the Mg(AI)O support interacts with Pt but not to the extent that tin does in a silica-supported catalyst with a Pt:Sn = 1 : 1 ratio. The kinetic isotope effect for Pt-Mg(Al)O is about 2 and is comparable to the one obtained with a Pt-Sn-SiO2 catalyst but is considerably lower than the value of 3.5 obtained with a Pt-SiO2 catalyst.