Single long n-alkanes in the range n-C-10 to n-C-24 were hydroisomerized at 233 degreesC in a fixed-bed down-flow vapor-phase reactor loaded with Pt/H-ZSM-22 zeolite catalyst. The multibranched isomer fractions obtained by skeletal isomerization. of the different n-alkanes were analyzed in detail with GC-MS. The large majority of multibranched isomers were dimethyl branched. In the preferred isomers, the tertiary carbon atoms carrying the methyl branchings were separated by 3 up to 14 carbon atom positions. Further peculiarities were the preferential formation of isomers with symmetric methyl positions on the chain. Among this family of "symmetric" isomers, the formation of 2,x - 3-dimethyl-Cx-2 isomer was favored in conversions of n-C-x alkanes with x equal to or smaller than 17 and disfavored with longer molecules. In the family of 3,n-dimethyl-Cx-2 isomers obtained from the longest alkanes studied (n-C-20, n-C-22, n-C-24), the preferred n positions were from 8 to 12 and from 14 to 18. In the 4,n-dimethyl-Cx-2 isomers, the preferred n positions were from 8 to 17. It was also found that the formation of a dimethyl-branched isomer occurs preferentially through methyl branching of a monomethyl-branched isomer having already the more centrally positioned methyl group rather than the one close to the extremity of the chain. These peculiar positional selectivities and reaction paths can be explained by key-lock catalysis in pairs of micropore openings on the external surface of the ZSM-22 zeolite crystals.