The voltammetric determination of quercetin in tea samples was performed at a modified sensor which was fabricated by the addition of a single-walled carbon nanotube dispersion on a glassy carbon electrode. Due to the electrochemcal activity of quercetin, electroanalytical methods were used with great success for the determination of quercetin. The electrochemical properties of quercetin were investigated. Quercetin in water and methanol extracts of tea samples were determined at the modified glassy carbon electrode by using square wave voltammetry. The results were found to be 116.4 and 116.3 mu g quercetin equivalent/g dry green tea, 127.9 and 104.24 mu g quercetin equivalent/g dry basil tea 27.5 and 26.1 mu g quercetin equivalent/g dry black tea. The limit of detection and limit of quantification values for quercetin for the 0.01 to 100 mu M concentration range were 7.7 nM and 25.6 nM, respectively. Very low concentrations of quercetin were successfully determined by square wave voltammetry with sensitivity and selectivity.