A kinetic method for studying the activity of medicinal plant extracts with respect to the stable 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical is developed. The initial rate of DPPH decay under standard conditions is suggested and theoretically justified as a kinetic parameter to compare the antiradical activity of extracts. A 10- to 150-fold decrease of the DPPH reaction rate with antioxidants (AO) of plant extracts in aqueous EtOH is achieved by adding acids to the reaction medium. The reaction at the optimum acid concentration decreases with increasing acid strength. Using this approach, the influence of organic acids extracted from the plant material on the results can be eliminated by adding stronger acids. The noise level on the kinetic curves and, therefore, in the measurement error, is decreased by smoothing the data using a general equation of the kinetic curve obtained using the Microcal Origin 7.0 and SYSTAT TableCurve 2D 5.01 software. It is found that the extent of DPPH conversion in the interval from 0 to 60% after the first 30 min depends linearly on the initial AO concentration.