Water activities of ammonium sulfate-sulfuric acid termiued relative to the known properties at RH about mixtures with ammonium to sulfate molar ratios be-0.75-0.90. The data were compared with other meatween 0 and 2 were measured by a spherical void surements and the estimates from the Zdanovskiielectrodynamic levitator at relative humidities (RH) of Stokes-Robinson (ZSR) method and SCAPE, a newly 0.18-0.90. Since the composition of the solid particles developed gas-particle equilibrium model, and generis subject to uncertainty, solution properties were deally were found to be in good agreement.