Activation rate constants (k(act)) for a variety of initiators for Cu-mediated ATRP have been determined under the same conditions. The ratio of the activation rate constants for the studied alkyl (pseudo)halides exceeds 1 million times. The activation rate constants increase with initiator substitution (e.g., for primary, secondary, and tertiary alpha-bromoesters the ratios are similar to 1:10:80), with the radical stabilizing alpha-substituent (e.g., alkyl bromides with -C(O)NEt2, -Ph, -C(O)OMe, and -CN groups the ratios are similar to 1:4:8:600 but with both alpha-Ph and alpha-C(O)OEt similar to 140 000), and with the leaving atom/group (e.g., for methyl 2-halopropionates: chloro:bromo:iodo similar to 1:20:35, but benzyl bromide is similar to 10 000 more reactive than the corresponding isothiocyanate/thiocyanate).