Osmotic coefficients of aqueous KCl solutions have been determined by the isopiestic method using standard NaCl reference solutions for comparison. Measurements were made at 283.15, 288.15. 293.15, and 298.15 K. A modified isopiestic apparatus is described. Equilibrium times ranged from 4 to around 15 days. The equilibrium molalities went up to 1.0 m. The average relative uncertainty of the observed eater activities of the solutions was less than 0.005%. The activity and osmotic coefficients of KCl aqueous solutions at several temperatures are compared with literature data. (C) 2003 Elsevier B.V. All rights reserved.