The osmotic coefficient of ZnCl2(aq) has been measured at T = 298.15 K by the isopiestic method over the range of molalities from (0.4 to 2.4) mol . kg(-1) with KCl(aq) as the reference solution. The available literature data on the osmotic coefficient of ZnCl2(aq) at T = 298.15 K, and new experimental data were treated by the Clegg-Pitzer-Brimblecombe equation and the extended Pitzer's equation. These equations were used to calculate the activity coefficient of ZnCl2 at T = 298.15 K. The formation of the ZnCl+ complex has been taken into account by means of the Clegg-Pitzer-Brimblecombe equation. (C) 2003 Elsevier Science Ltd. All rights reserved.