Diffusion coefficients of supersaturated aqueous solutions of ammonium sulfate have been measured at 298 degrees C using Gouy interferometry and were observed to rapidly decrease with increasing concentration in the supersaturated region. Water activities in the supersaturated region were measured by electrodynamic levitation of single solution droplets. The number average cluster size, critical cluster size, degree of association and spinodal concentration were calculated employing these data. These results show that the true metastable zone width is much wider than previously thought with a calculated valve of the spinodal concentration at 69.63 molal.