The Wagner-Pruss equation is modified for expressing the vapor pressures of aqueous KCl and MgCl2 solutions from 273.16 to 643.15 K. The salt concentrations are up to 20.7mass% for the H2O +KCl system and 20.9mass% for the H2O+MgCl2 system. This study adds the concentration-dependent terms to the Wagner-Pruss equation and multiplies the reduced temperature by T-c/647.096, where T-c is the critical temperature of the aqueous solution in Kelvin. (C) 2004 Elsevier B.V. All rights reserved.