Flavonoids are a large group of secondary plant metabolites with hydroxyl groups. The flavonol quercetin is commonly found in onions. In the edible fleshy scales of yellow onions quercetin mono- and diglucosides are present. The aim of this work was to study quercetin glucosides during technological processing of yellow onion bulb. Onion was boiled in water under reflux for 30 min. The cooking was performed at three different onion/water ratios (1/5, 1/10, 1/20; w/w). The resulting soup, boiled onion solids and the control raw onion samples were lyophilised, extracted with ac. methanol and analysed for quercetin and its derivatives (3-β-D-glucoside, 4’-β-D-glucoside, 3,4’-β-D-glucoside) using HPLC-UV. Major flavonoid components identified in yellow onion were quercetin-4’-β-D-glucoside
and 3,4’-β-D-glucoside with a decreasing concentration from outer toward inner rings of the raw bulb. These substances were rapidly transferred into cooking water during thirty minutes boiling making the resulting soup a good source of flavonoids. The effect of water amount added on flavonoid concentration was followed.