The complex formation between cyclodextrins (CDs) and poly(propylene glycol) (PPG) derivatives is described. beta -CD and gamma -CD formed complexes with PPG derivatives such as l-naphthyl (1NA), a-naphthyl (2NA), 3,ti-dinitrobenzoyl, and 2,4-dinitrophenyl PPG. alpha -CD did not form complexes with these PPG derivatives. Although gamma -CD gave complexes with 9-anthryl PPG (PPG9An), beta -CD did not efficiently form complexes with PPG9An. beta -CD did not form complexes with trityl PPG, demonstrating that trityl groups were too bulky to thread a beta -CD cavity. The emission spectra of the complexes showed that beta -CD bound a single 2NA moiety in its cavity and that gamma -CD included two 2NA moieties. In contrast, gamma -CD bound a single 1NA moiety in the cavity. X-ray diffraction studies and H-1 NMR analysis showed that the CD molecules were stacked along a PPG; chain to form a channel structure. The inclusion modes are discussed.