Blood was collected from six human volunteers 1h after the ingestion of lightly fried red onions, which are rich in anthocyanins and flavonols. Urine excreted 0–4h after ingestion was also collected. Plasma and urine were analysed by reversed-phase HPLC with photodiode array and tandem mass spectrometric detection. Twenty-three flavonols as a range of mixed sulphate, methyl, glucuronide and glucoside derivatives of quercetin were detected. The availability of reference compounds facilitated the identification of quercetin-3-glucuronide, quercetin-3′-glucuronide, quercetin-4′-glucuronide, quercetin-3′-sulphate and isorhamnetin-3-glucuronide in samples from all volunteers. Samples from one volunteer also contained trace amounts of quercetin-3,4′-diglucoside, quercetin-3-glucoside, isorhamnetin-3-glucoside and the aglycone quercetin. Despite a high dosage, neither anthocyanins nor anthocyanin metabolites accumulated in either plasma or urine in detectable quantities.