Significant discrepancies of literature data on the quercetin (Q) water solubility (the variations exceed 5000 times!) at the ambient temperature prompted the elaboration of a new approach on its determination. The new approach is based not on the single measurements, but on the revealing and comparing the two dependencies of solubility. First of them is the dependence of the solubility on pH of water solutions S (Q) = a× pH + b , followed by S (Q) interpolation on pH = 7, which corresponds to the pure water. The second one is the dependence of the solubility on acetonitrile content in acetonitrile-water mixtures, log S (Q) = a× [CH3CN] + b , followed by log S (Q) extrapolation on the zero concentration of organic solvent, which corresponds to the pure water as well. The analysis of the solutions was fulfilled using the reversed phase high performance liquid chromatography (RP HPLC). The results obtained independently with various methods correspond well with each other: their joint average value for the few samples was 7.1 ± 1.6 mg L-1 (p S = 4.63). Since the results were obtained for quercetin samples that contained different amounts of crystalline and hydrate water, the final sub-routine of analytical procedure was their extrapolation on the zero-water content in the quercetin samples. The necessity of the preliminary control of both the water content and the organic impurities in the samples using elemental and HPLC analysis was discussed. The samples of quercetin, as well as other plant extractive substances, could contain impurities that distort the results of its water solubility.