The oxidative degradation of quercetin and rutin in phosphate buffer solutions, pH 8.0, at 97 degrees C, was studied by means of UV-vis spectroscopy and reversed-phase high-performance liquid chromatography (HPLC). The effect of the transition metal ions Fe(2+) and Cu(2+) on degradation rate and browning development was also assessed. It was shown that both flavonols are very labile to thermally induced degradation under oxidative conditions. Fe(2+) and Cu(2+) caused an increase in the degradation rate, as well as an increase in browning (A(420)). Significant differences were observed in the degradation mechanisms, as implied by HPLC analyses. It is postulated that metal ions promote flavonol oxidation through reactive oxygen species formation, whereas increases in browning could be ascribed to oxidation and metal-polyphenol interactions.