We studied the conversion of methylcyclohexane into dimethylpentanes in the presence of hydrogen, catalyzed by It supported on H-beta zeolites with different Si/Al ratios. Ir/H-beta catalysts with a high Si/Al ratio (Si/Al = 75 or 150) showed activity in promoting both ring-contraction (from six-membered to five-membered rings) on the acidic sites of H-beta and ring-opening on Ir metal sites, resulting in high activity and selectivity toward dimethylpentanes. In contrast, Ir/H-beta catalysts with a low Si/Al ratio were less active in ring-opening on the Ir metal sites, which resulted in the high selectivity for dimethylcyclopentanes. In the conversions of methylcyclohexane, 2-methylhexane, and 2,4-dimethylpentane, the dependence of the conversion rate on W/F values demonstrated that, for the highly active and selective Ir/H-beta catalyst with Si/Al = 75, methylcyclohexane was first transformed into dimethylcyclopentanes on the acidic sites, followed by ring-opening on the Ir sites to form dimethylpentanes and methylhexanes. The skeletal isomerization of methylhexanes into dimethylpentanes over this catalyst was very slow under the reaction conditions studied. (C) 2006 Elsevier B.V. All rights reserved.