Binding properties of poly(tetrahydrofuran) (PTHF) to methylated cyclodextrins (CDs) in aqueous solutions were studied by turbidity titration and H-1 NMR measurements. PTHF, which has a low solubility in water, was dissolved in water on addition of 2,6-di-O-methyl-beta-CD (DM-beta-CD) and DM-alpha-CD. 2,3,6-tri-O-methyl-beta-CD (TM-beta-CD) and TLM-alpha-CD did not solubilize PTHF. When PTHF was mixed with aqueous solutions of DM-beta-CD and TM-beta-CD, the inclusion complexes crystallized after several days. 1,4-Butanediol (monomer model) and PTHF (M-w = 250) did not form crystalline complexes with methylated CDs. The yields of the complexes of PTHF with methylated beta-CDs increased with increasing molecular weight of PTHF and reached a maximum at about M-w = 1000. The complexes were characterized by 1H NMR, solid-state C-18 NMR, powder X-ray, and FT-IR measurements.