The supramolecular structure of the aggregate of chlorophyll a＇ (Chl a＇, the C13(2)-(S)-epimer of Chl a) was examined in comparison with that of the Chl a aggregate. The Chl a/a＇ aggregate colloids were formed in aqueous alcohols, and IR, resonance Raman, and small-angle X-ray scattering (SAXS) measurements were performed on lyophilized precipitates of the aggregate colloids. The analyses on the Chl a colloidal precipitate obtained from a 26/74 2-propanol/H2O solution demonstrated that the supramolecular structure was similar to those of the aggregates of hydrated Chl a. On the other hand, the vibrational spectra of the Chl a＇ colloidal precipitate formed in 40/60 MeOH/H2O were quite similar to those of anhydrous Chl a aggregates. A trace hydroxyl stretching IR absorption (at around 3350 cm(-1)) of the Chl a＇ aggregate precipitate was as small as that which the anhydrous Chl a aggregates could show. A SAXS profile of the Chl a＇ colloidal precipitate demonstrated a lamellar structure with a 52-Angstrom bilayer spacing, 8 Angstrom greater than that of the Chl a aggregate formed in the 2-propanol/water. It was supposed that the Chl a＇ aggregate was essentially anhydrous even in aqueous alcohols, while the Chl a aggregate was hydrated or swollen. A possible model of the supramolecular structure of the Chl a＇ aggregate precipitate is presented. It is concluded that the difference between the supramolecular structures of the Chl a and a＇ aggregates does not depend simply on the steric hindrance between the bulky substituents at the C13(2) and C17 positions but also on the possibility to form rigid intermolecular hydrogen-bonding networks. This is the origin of the diastereoselective separation of Chl a and a＇, and it is also closely correlated to a structural degradation of the Chl a＇ aggregate that occurs during the preparation of the colloidal precipitate.