The ring opening (RO) of 1,3-dimethylcyclohexane (DMCH) has been studied on a series of Ir catalysts. This molecule has been used as a model feed to compare catalysts that could be employed in the treatment of gasoline feedstocks for improving octane number (ON), while keeping low aromatics content and low vapor pressure. By changing the support and by adding promoters, we have been able to modify the product distribution and determine which catalyst modifications can be more effective. For example, Ir/SiO2 catalyzes the opening of unsubstituted C-C bonds and yields products with a high degree of branching and consequently high ON. By contrast, Ir/Al2O3 is less selective and yields products with lower ON. This catalyst is at the same time more active and results in excessive secondary hydrogenolysis, producing increasing amounts of light-molecular weight products as the conversion increases. K and Ni were investigated as potential promoters to inhibit the secondary hydrogenolysis and make the Ir/Al2O3 catalyst more selective towards the cleavage of unsubstituted C-C bonds. It was found that while the addition of K decreased the secondary hydrogenolysis it did not change the ratio of substituted/unsubstituted C-C bond cleavage and had little effect in the resulting octane number of the product. By contrast, the addition of Ni was found to improve octane number without excessively increasing the vapor pressure of the mixture. (C) 2007 Published by Elsevier B.V.