The thermodynamic properties of some copper complexes, among those frequently used as catalysts in controlled/living radical polymerization, has been studied in CH3CN + 0 1 M (C2H5)(4)NBF4 A combination of different techniques namely potentiometry spectrophotometry and cyclic voltammetry, has been used to determine the stability constants of all possible complexes of Cu-I and Cu-II present in binary and ternary systems composed of Cu-I or Cu-II, a halide ion (X = Cl-, Br-) and a polyamine ligand (L = pentamethyldiethylenetriamine, tris(2-dimethylaminoethyl)amine) The binary Cu-X systems show only mononuclear CuXx complexes, where x = 1, 2, 3, 4 for Cu-II, and x = 1, 2 for Cu-I Conversely, in the case of the binary Cu-L systems besides the mononuclear complexes CuLl, where l = 1 or 2 also dinuclear complexes Cu2L were found The ternary systems give rise to a mixture of mononuclear and dinuclear complexes of general formula CumLlXx Besides the 1 1 1 complex obtained in all combinations, the following species were found (CuLX2)-L-II (Cu2LX)-L-I and (Cu2LX2)-L-I The stability constants of all these species were determined and used to construct speciation diagrams for both Cu-I and Cu-II species Such diagrams show that often conditions favoring the quantitative formation of (CuL)-L-II, (CuL2)-L-II, or (CuLX)-L-II can be easily realized, whereas isolation of a single predominant Cu-I species can hardly be achieved Speciation diagrams for Cu-I as a function of C-X/C-CuI show interesting results that may be helpful in rationalizing the role of termination reactions in atom transfer radical polymerization