beta-Cyclodextrin (beta-CD) and gamma-cyclodextrin (gamma-CD) form inclusion complexes with poly(propylene glycol)s (PPG) of various molecular weights to give stoichiometric compounds in crystalline states. alpha-Cyclodextrin (alpha-CD) did not form complexes with PPG of any molecular weight. beta- and gamma-CD did not form complexes with the low molecular weight analogs, such as propylene glycol, di(propylene glycol), and tri(propylene glycol). The yields of the complexes of beta- and gamma-CD with PPG increased with increasing molecular weight (MW) of the PPG and reached a maximum at about MW 1000; yield decreased with a further increase in the MW. They were isolated and found to be 2:1 (monomer unit:CD). The complexes were characterized by IR, H-1 NMR, C-13 NMR, C-13 CP/MAS NMR, and C-13 PST/MAS NMR spectra and X-ray (powder), thermal, and elemental analyses. Complex formation of CDs with PPG derivatives has also been studied. The structures of the complexes are discussed. Complex formation of beta-CD with atactic and isotactic PPG has been compared.