Adsorption of various octanes and octanes was studied with a 10-membered ring molecular sieve, H-[B]ZSM-5. Adsorption of octane was very fast, the double-branched octane diffused slowly, and the triple-branched molecules could not enter the channels of the sample. Since the 10-membered ring molecular sieves are the best catalysts for skeletal isomerisation of n-butene, trimethylpentenes or other larger oligomers cannot be intermediates in this reaction. By contrast, skeletal isomerisation of n-butane which is known to require 12-membered ring channels, can involve a trimethylpentane-like molecule as an intermediate.