The extraction of copper(D) from concentrated chloride ion solutions by various pyridinecarboxylic acid esters is investigated. It is shown that the efficiency of the extraction is strongly dependent on the water activity (aw) and total concentration (σ) of ionic or molecular species dissolved in the aqueous phase. For decyl nicotinate, decyl isonicotinate, decyl picolinate and ACORGA CLX-50 (all denoted EXT) in toluene, the distribution of copper(II) can be satisfactorily represented by the formation of CuCl2(EXT)2in the organic phase on the condition of taking into account the existence of CuClx(2-X)complexes (x = 1 to 4) in the aqueous phase. The apparent extraction constant KeX and stability constants |βi of the variousCuOx(2-X)complexes are estimated for two aqueous media corresponding to awandσ= 0.835 and a = 8.0 mol.L-1, and aw= 0.617 and σ = 12.0 mol.L-1, respectively. On the other hand, in the case of ACORGA CLX-50 in kerosene, more complex phenomena influencing the activity coefficient of the extractant seem to occur.