A solid precipitate is formed immediately upon the mixing of aqueous salt solutions of the fluorescence probe 1-anilino-8-naphthalenesulfonate (1,8-ANS) and the cage compound cucurbituril. The resulting solid compound is highly fluorescent, with a maximum emission at ca. 478 nm. Single crystals of this compound were obtained and studied by X-ray crystallography. The crystal structure indicates a rather novel solid fluorescent compound, described as a lattice inclusion compound, in which the smaller 1,8-ANS molecules have cocrystallized in the lattice of the cucurbituril cages. This structure is in effect a highly ordered solid solution, in which the fluorescent I,8-ANS molecules are well separated from each other by the larger cucurbituril molecules. This separation of the fluorescent probe molecules by the nonabsorbing (in the visible range) cucurbituril spacers prevents interaction of the 1,8-ANS fluorophores and results in a highly fluorescent solid.