Conducting an atom transfer radical polymerization (ATRP) in water provides several intriguing challenges. In order to attain a deeper understanding of the mechanism of aqueous ATRP, with particular attention to the reasons for the potential loss of control, a systematic investigation was carried out with three copper-amine catalysts([(CuL)-L-II](2+), L = PMDETA, TPMA and Me6TREN). The association constants (K-x) of [(CuL)-L-II](2+) and [(CuL)-L-I](+) with X- and OH-, as well as the relative stabilities and redox properties of the complexes, were determined as a function of both [X-] and pH. In addition, rate constants and equilibrium constants of disproportionation for all three [(CuL)-L-I](+) complexes were measured. Finally, ATRP equilibrium constants (K-ATRP) were estimated for all complexes, using 2-hydroxyethyl 2-bromoisobutyrate (HEBiB) as initiator. The primary factors affecting control in aqueous ATRP were identified as catalyst instability at both low and high pH, easy dissociation of deactivator (K-x < 15), very high K-ATRP and fast disproportionation of [(CuL)-L-I](+) Overall, the results obtained in this study provide a set of guidelines for a well-controlled aqueous ATRP. The guidelines were used to conduct several efficient electrochemically mediated aqueous ATRP experiments with oligo(ethylene oxide) methyl ether methacrylate as monomer, HEBiB as initiator and copper complexes with three different ligands. A successful ATRP in a strongly acidic environment (pH 1.5) is reported for the first time, contrary to what was believed until now, indicating that ATRPs can be easily controlled in acidic conditions, while high pH should be avoided.