Background: Quercetin (QC) is a poorly water-soluble and degradation-prone bioflavonoid. Aim: This study aimed to investigate the formation of a chemically stable and readily water-soluble solid complex of QC with hydroxypropyl - cyclodextrin (HP - CD). Method: A solid HP - CDQC complex was prepared by spray drying and characterized by high-performance liquid chromatography, Fourier-transform infrared spectroscopy, scanning electron microscopy, laser-diffraction particle sizing, powder X-ray diffraction, thermal analysis, BrunauerEmmettTeller nitrogen adsorption, and dissolution testing. Results: The complex displayed excellent thermal stability during spray drying, a broad diffuse X-ray diffraction pattern, and a single glass transition temperature (Tg) characteristic of amorphous HP - CD. Dissolution testing of the complex in simulated gastric fluid revealed an approximately eightfold increase in the rate and extent of dissolution of QC compared with its molar equivalent physical mixture of QC and blank spray-dried HP-CD, although the surface area of QC in the mixture was about seven times that of the complex. Conclusions: All these observations can be explained by specific interaction between QC and HP - CD in the amorphous solid state, possibly in the form of an inclusion complex. © 2009 Informa UK Ltd.