Monodisperse oligo(ethylene glycol)s (OEG) have been prepared by stepwise synthesis coupled with preparative size-exclusion chromatography (SEC). Complex formation between alpha-cyclodextrin (alpha-CD) and the oligo(ethylene glycol)s was studied. Alpha-CD was found to form complexes with tetrakis(ethylene glycol) (TEG) and larger oligo(ethylene glycol)s. The yields of the complexes increase with an increase in the degree of polymerization. Eicosakis(ethylene glycol) and larger oligo(ethylene glycol)s formed complexes with alpha-CD almost quantitatively. The H-1 NMR spectra of the complexes show that the stoichiometry of the complexes is 2:1 (two ethylene glycol units and one alpha-CD) when the degree of polymerization is higher than 6. The stoichiometry of the complexes of alpha-CD with tetrakis(ethylene glycol) and pentakis(ethylene glycol) (PEG) is 2:1 (CD:OEG). The X-ray powder patterns of the alpha-CD-OEG complex show that alpha-CDs form channels. The C-13 CP/MAS NMR spectra of the complexes suggest that an OEG chain is included in the channel formed by alpha-CDs. Beta-CD did not form complexes with any OEG. Cyclic oligomers of ethylene glycol (crown ethers, 15-crown-5, and 18-crown-6) did not form complexes with alpha-CD, except for 12-crown-4 which gave complexes with alpha-CD in low yield. The modes of the complexes are discussed.