This work reports the study of the chemical equilibria of the extraction and back-extraction of zinc from spent pickling solutions. Tributyl phosphate was used as a selective extractant and service water as a back extraction agent. Feed solutions had an average composition of 6.4 M Cl- , 1.2 M Zn+2 , 1.7 M Fe+2 and traces of other heavy metals i.e., Pb, Ni, Cu, Mn. Under the experimental conditions the ion ZnCl42-was the predominant zinc species in feed. A careful experimental design was performed obtaining the equilibria mathematical models and parameters, K-EX =0.137 (mol/L)(-4) , and K-BEX =1.46 [-].