Oligomerization results of propene, 1-decene, and isobutene catalyzed by zeolite HZSM-5 and zeolite HZSM-5 surface deactivated with 2,6-di-tert-butylpyridine show that faster oligomerization and a higher percentage of C-20(+) products are obtained with the unmodified HZSM-5. Results of C-13 NMR analysis show that when the oligomerization takes place inside ZSM-5 channels, the structures of the C-20(+) products are nearly linear, with a small amount of methyl branching. A higher degree of methyl branching is obtained at higher reaction temperatures, The C-20(+) products obtained from different olefins with surface-deactivated HZSM-5 under the same experimental conditions have nearly identical structures. The near-linear structure of the product indicates that when the oligomerization reaction takes place inside HZSM-5 channels, most of the alkyl substituents at the double bond of the olefins become part of the polymer chain, due to the steric requirement. When the surface acidity is unmodified, most oligomerization reaction takes place at the outer surface of the zeolite, and complex product structures result.