A new electrochemical procedure is described to determine the equilibrium constant of atom transfer radical polymerization, K-ATRP. A rotating disk electrode (RDE) was used to monitor the concentration of the catalyst, (CuL+)-L-I, during a reaction between the metal complex and an alkyl halide (RX) used as a polymerization initiator. K-ATRP values with errors in the range +/- 10% were measured. Conditions were defined to avoid undesired reactions such as catalytic radical termination and competition between RX and X- for the reactive site of copper. The RDE technique was also used to determine the equilibrium constant for association of X- to (CuL+)-L-I, K-X(I), a parameter hardly accessible by other methods. The RDE technique was applied to different ligands, initiators, solvent/monomer combinations, and reaction temperatures. Linear correlations between logK(ATRP) and logk(act) and logk(deact) were found. (C) 2017 Elsevier Ltd. All rights reserved.