Subcritical water could be an excellent alternative to organic solvent as a medium for extracting flavonol quercetin, due to its temperature-dependent selectivity, safety, efficiency of recovery, and lower cost. This study investigated the application of subcritical water extraction (SWE) of quercetin from onion skin and evaluated the effect of key operation conditions by varying the temperature (100-190 degrees C), extraction time (5-30 min), and mixture ratio of onion skin and diatomaceous earth (DE) (0.5:3.5-2:2) under high pressure (90-131 bar). The maximum yield of quercetin (16.29 +/- 0.75 mg/g onion skin) was obtained at extraction temperature of 165 degrees C, extraction time of 15 min, mixture ratio of 1.5:2.5 for onion skin and DE. The SWE was compared with three conventional extraction methods in terms of the efficiency. The quercetin yield by SWE was over eight-, six-, and fourfold greater than those obtained using the ethanol, methanol, and water-at-boiling-point extraction methods, respectively. (C) 2010 Elsevier Ltd. All rights reserved.