A detailed study on the effect of [PMDETA]/[Cu(I)] ratio, monomer, solvent polarity, counterion, ligand, and alkyl bromide on the activation rate constant (k(act)) in ATRP was carried out. The highest values of k(act) for Cu(I)Br were obtained at [PMDETA]/[Cu(I)Br] similar to 1/1 in more polar solvents and ascribed to a neutral [Cu(PMDETA)Br] structure of the complex. However, in less polar solvent mixtures a relatively fast reaction was observed already at the 0.5/1 ratio with the smaller rate increase up to [PMDETA]/[Cu(I)Br] similar to 1/1. The highest values of k(act) for Cu(I)PF6 were observed at [PMDETA]/[Cu(l)Br] similar to 1/1 in more polar, less polar solvent mixtures and monomer, which was explained by the formation of an ionic [Cu(PMDETA)]+PF6- complex. In both more polar and less polar media, the values of k(act) were slightely larger for PF6- than Br- counterion. However, in methyl acrylate, k(act) was 1.9 times larger for Br- than PF6-counterion. This was attributed to monomer coordination through the open coordination site of an ionic [Cu(PMDETA)]Y-+(-) (Y = PF6/BPh4) complex, which reduced complex activity. The values of k(act) were higher for PMDETA containing three nitrogen atoms as compared to the ligand containing two nitrogen atoms and one oxygen atom (BDMAEE). The relative rates of k(act) of the alkyl bromides follow the order EBriB (30) >> MBrP (3) > t-BBrP (1). In the absence of monomer, the values of the k(act) for different ligand/counterion follow the order PMDETA/PF6- > PMDETA/Br- > BDMAEE/Br- > bpy/ PF6- > bpy/Br-.