The osmotic coefficient of ZnSO4(aq) has been measured at T = 298.15 K by the iso-piestic method, in the range of molalities from 1.3 to 3.0 mol . kg(-1), with KCl(aq) as the reference solution. Experimental data of osmotic coefficient were treated to determine the ionic interaction parameters in Pitzer's equation and equation of Clegg-Pitzer-Brimblecombe. These equations were used to calculate the activity coefficient of ZnSO4 at T = 298. 15 K.