Fundamental physicochemical data is required for the design and optimization of food engineering processes, such as extraction. Flavonoids are present in natural products such as grapes and have numerous health benefits particularly with respect to their reported antioxidant properties. Such flavonoid compounds can be extracted from these natural products using a variety of solvents, among them water. In this study, the aqueous solubilities of 3,3',4',5,7-pentahydroxyflavone (quercetin) and its dihydrate were measured at temperatures between 25 and 140 degrees C using a continuous flow type apparatus. The flow rate of subcritical water was studied at 0.1, 0.2 and 0.5 mL/min to study its effect on quercetin solubility and thermal degradation at temperatures greater than 100 degrees C. The aqueous solubility of anhydrous quercetin varied from 0.00215 g/L at 25 degrees C to 0.665 g/L at 140 degrees C and that of quercetin dihydrate varied from 0.00263 g/L at 25 degrees C to 1.49 g/L at 140 degrees C. The aqueous solubility of quercetin dihydrate was similar to that of anhydrous quercetin until 80 degrees C. At temperatures above or equal to 100 degrees C, the aqueous solubility of quercetin dihydrate was 1.5-2.5 times higher than that of anhydrous quercetin. The aqueous solubility of quercetin anhydrate and dihydrate at different temperatures was correlated using a modified Apelblat equation. The thermodynamic properties of the solution of quercetin and its dihydrate in water were than estimated from their solubility values. A flow rate effect on the aqueous solubility of quercetin and its dihydrate was not observed until above 100 degrees C where higher solvent (water) flow rates (>0.1 mL/min) were required to maintain a constant solubility in the saturation cell and with minimal thermal degradation of the solute (quercetin dihydrate). The study of its particle morphology under SEM indicated an aggregation of the crystals of quercetin dihydrate at subcritical water temperatures and at lower flow rates (<0.5 mL/min), thereby inhibiting stable solubility measurements and solvent flow through the saturation cell. (C) 2010 Elsevier Ltd. All rights reserved.