This paper reports a study of the extraction equilibrium of zinc from sulfate media with bis(2-ethylhexyl)thiophosphoric acid. The distribution results were interpreted by taking into account the nonideality of the aqueous phase. The activity coefficients of the ions in the ZnSO4/H2SO4/Na2SO4 aqueous system were calculated by applying the Pitzer model. Complete dissociation of the sodium salt was found to be a suitable assumption. The presence of the NaSO4- species did not significantly affect the thermodynamic results. The extraction of zinc from sulfate solutions with bis(2-ethylhexyl)thiophosphoric acid can be described by the overall stoichiometry Zn2+ + 1.5(HR)(2) reversible arrow ZnR2.HR + 2H(+). The thermodynamic equilibrium constant in terms of the aqueous phase was also evaluated and was found to be 32 M-0.5 at 293 K.