Acid-dissociation of heptakis(2,3-di-O-carboxymethyl)-beta-cyclodextrin occurred between pD 2 and 6. At pD 2.0, 1-pyrenesufforiate was included into the host cavity with a binding constant of 2300 +/- 100 M-1, while no complexation occurred at pD > 6.0. Much sharper pH-dependency in complexation proceeded with 1-anilino-S-naphthalenesulfonate and 2-(p-methylanilino)-6-naphthalenesulfonate.