1-Cyclohexyloctane is isomerized in the vapor phase on Pt/H-ZSM-22 zeolite catalyst in the presence of hydrogen. Most of the reaction products are identified and analyzed using high-resolution GC and GC/MS. The reaction pathways of 1-cyclohexyloctane are interpreted in terms of alkylcarbenium/alkylcarbonium ion chemistry. The main isomerization pathways are branching of the ring, elongation and shortening of alkyl substituents on the ring, and ring contractions and expansions. A less important pathway is chain branching, occurring selectively at C-atom positions distant from the ring. In the reaction product fraction of heptyl, methylcyclohexanes, preferential formation of the 1-heptyl-t-4-methylcyclohexane is observed. The formation of l-heptyl, Il-methylcyclohexane is completely suppressed. The peculiar selectivity patterns are explained by a mechanism in which the molecules react in micropore mouths at the external surface of the ZSM-22 zeolite crystals.