beta-Cyclodextrin (beta-CD) and gamma-cyclodextrin (gamma-CD) formed inclusion complexes with poly isobutylene (PIE) of various molecular weights to give stoichiometric compounds in crystalline states. alpha-Cyclodextrin (alpha-CD) did not form complexes with PIE of any molecular weight. The yields of the complexes with beta-CD decreased with an increase in the molecular weight of PIE. In contrast, the yields of the complexes with gamma-CD increased with an increase in the molecular weight and the complexes were obtained almost quantitatively with PIE of molecular weight 1350. The chain-length selectivity is reversed between beta-CD and gamma-CD. The inclusion complexes were isolated and found to be 3:1 (monomer unit:CD). The complexes were characterized by IR, H-1 NMR, C-13 NMR, C-13 CP/MAS NMR, C-13 PST/MAS NMR spectra, and X-ray (powder), thermal, and elemental analyses. The structures of the complexes are discussed.