Communication: Two geometrically isomeric gamma-cyclodextrin derivatives with a thymolphthalein moiety in the secondary hydroxyl side were prepared as guest-responsive color change indicators. The isomers exhibit a pH dependence of their absorption spectra and a remarkable change in the absorbance around 610 nm upon guest addition. One of the isomers formed complexes of 2: 1, the other of 1 : 1 stoichiometry (host/guest).