Quercetin is a flavonol, a subclass of flavonoids that has numerous pharmacological uses due to its antioxidant, anticarcinogenic, antiallergic and anti-inflammatory properties. The reaction of flavonoids and DPPH• are commonly monitored by measuring the decrease in absorption at the wavelength corresponding to the maximum DPPH•. According to new literature (Foti et al., 2011), the decrease in absorption in the visible region of the spectrum is not directly related to the reduction of DPPH • and the reaction solution, except initially, does not contain the DPPH radical. In this thesis, the reaction kinetics of quercetin and DPPH• in a solvent mixture of dioxane-water (0,99:0,01 v/v) and methanol were investigated. Based on the measured dependence of the pseudo-first order reaction rate constant on the quercetin concentration, the reaction rate constant of quercetin and DPPH • in dioxane was determined to be 4,38 mol-1dm3s-1. The rate constants determined at 525 nm and 620 nm are approximately the same, although the deviations in the measured values at 525 nm are significantly larger compared to 620 nm. The reaction rate constant of quercetin and DPPH• in methanol with a value of 469 mol-1dm3s-1 was determined. No deviations in the measured values were found in methanol as in dioxane. Based on the measured dependence of the pseudo-first order reaction rate constant on the quercetin concentration in the solvent dioxane-D2O, a reaction rate constant kD of 0,568 mol-1dm3s-1 and a kinetic isotopic effect of 7,71 were determined. These data were not previously found in the literature and were determined for the first time in this thesis. Both kinetic and HPLC measurements of catechin and DPPH • reactions were performed for comparison. The rate constant of the reaction of catechins and DPPH• in methanol was determined to be 2,70 mol1dm3s-1. HPLC analysis confirmed that DPPH• (in the reaction of quercetin and DPPH • and the reaction catechin and DPPH •) was present in the reaction solution during the whole period in which the decrease in absorption was monitored in the visible part of the spectrum.