Dedicated to Professor Andre¬ M. Braun on the occasion of his 60th birthday Equations are derived to quantitatively describe the effect of a free-radical scavenger upon the rate of a radical-mediated process that senses the steady-state free-radical concentration. The dependence of the ratio R8/R (where R8 is the rate of the process in the absence of additive) upon the additive concentration depends upon the type of reaction that determines the free-radical lifetime. Normal Stern-Volmer-like behavior is expected only when the lifetime of the radical in the absence of free-radical scavengers is determined by the concentration of the substance employed as the reporter of the free-radical concentration. These predictions are tested in a system comprised of 2,2'-azobis[2-methylpropanimidamide dihydrochloride) as the free-radical source, c-phycocyanin as the reporter molecule, and Trolox ( 3,4-dihydro-6-hydroxy-2,5,7,8-tetramethyl-2H-1benzopyran-2-carboxylic acid) tryptophan and 4-methoxyphenol as peroxyl-radical scavengers. The data obtained with Trolox show that it behaves as a nearly ideal free-radical scavenger. On the other hand, the data obtained with tryptophan and 4-methoxyphenol as scavengers show, when plotted according to the Stern-Volmer equation, a strong downward curvature. These results are explained in terms of c-phycocyanin bleaching by scavenger-derived free radicals.