A novel beta-cyclodextrin (beta CD) inclusion compound was prepared using 4-phenylpyridine-N-oxide (PPNO) as the organic guest. The inclusion compound, beta CD.PPNO, was characterised both in solution and in the solid state using numerous techniques. H-1 NMR in aqueous solution allowed the determination of a 1 : 1 stoichiometry and an association constant of 164 +/- 21 M-1. Powder and single-crystal X-ray diffraction studies showed the formation of two distinct crystal phases, appearing at different timings. Over time, one of the crystal phases converts spontaneously into the other. This work is the first to monitor the conversion of different polymorphs of cyclodextrin inclusion compounds in real time.