This study investigates the mechanism of copper(I)-mediated "living" atom-transfer radical polymerization (ATRP) in aqueous media. It is shown that the ATRP apparent rate constant for polymerization of methoxycapped oligo(ethylene glycol) methacrylate (OEGMA) in water (k(P)(app)) at room temperature correlates with the redox potential (E-1/2) of the copper complexes. The results are discussed along with previously published results on the kinetics for bulk polymerization of methyl acrylate at 60 degreesC with the redox potentials measured in MeCN. The faster ATRP kinetics in water can mainly be attributed to a higher equilibrium concentration of propagating radicals [W] and to solvent effects on the rate of propagation k(p). It is shown that [R*] can be calculated from the redox properties of the alkyl halide and the copper complex. The values of [R*] in MeCN/bulk and in H2O were determined to be 8.2 x 10(-8) and 6.3 x 10(-5) M, respectively. The respective kp values are in good agreement with the literature values (3.6 x 10(3) M-1 s(-1) for OEGMA in water and 2.5 x 10(3) M-1 s(-1) for methyl acrylate in bulk).