The rate constants of deactivation in atom transfer radical polymerization of styrene and butyl acrylate have been determined. The values are k(deact) = 7 x 10(7) L mol(-1) s(-1) for styrene and 1 x 10(8) L mol(-2) s(-2) for butyl acrylate, both measured in p-xylene at 110 degreesC. The effect of solvent polarity seems to be insignificant on the basis of results measured in butyl acetate. The proper conditions for the experimental determination of k(deact) were found on the basis of computer simulations. Conditions were chosen such that the sensitivity of the measured rate constants toward experimental conditions and toward uncertainties in other rate parameters was minimal.