Octadecane was isomerized over three different Pt/H-ZSM-23 samples. The distributions of methylheptadecane and dimethylheptadecane skeletal isomers obtained on the Pt/ZSM-23 samples were very similar. Positional isomer distributions are fingerprints of the zeolite framework structure. The independence of skeletal isomer distributions from Al-gradients and particle size constitutes a strong argument in favor of pore-mouth catalysis.