Atom transfer radical polymerization (ATRP) in water is characterized by very high catalytic activity, i.e. the rate constant (kact) of the "activation" reaction between Cu-I complexes and alkyl halides (RX) is very large. To accurately study such fast reactions in water, we developed a collection of electrochemical methods applicable for the determination of a vast range of kact values (>7 orders of magnitude). These included rate constants of extremely fast reactions with k(act) > 10(7) mol(-1) dm(3) s(-1). We studied three widely used ATRP catalysts ([Cu(I)PMDETA](+), [Cu(I)TPMA](+), and [(CuMe6TREN)-Me-I](+)) and an extended series of water-soluble initiators. Accurate results were obtained in the range 1 < k(act) < 10(8) mol(-1) dm(3) s(-1), allowing to build for the first time precise structure reactivity correlations for aqueous ATRP, in both pure water and water/monomer mixtures. The results were compared to traditional ATRP systems in organic solvents. The collected data greatly expand the available range of investigated catalyst/initiator systems.