The dehydration and the superprotonic transition behavior of Cs 0.75 Rb 0.25 H 2 PO 4 have been investigated via thermogravimetry and differential thermal analysis. To enable measurements under the high water partial pressures required, a high humidity system (pH 2 O =0.02–0.8atm) capable of weight change detection at 0.7% resolution and thermal events with 0.01°C resolution was assembled. It has been found that the superprotonic phase transition temperature of Cs 0.75 Rb 0.25 H 2 PO 4 is 231°C, slightly higher than that of CsH 2 PO 4 . In contrast, dehydration of Cs 0.75 Rb 0.25 H 2 PO 4 under pH 2 O =0.35atm occurs at 265°C, a somewhat lower temperature than that at which CsH 2 PO 4 undergoes dehydration. Analysis of the temporal evolution of both transformations suggests a mechanism of continuous nucleation of the high temperature phase at a constant nucleation rate combined with three-dimensional growth.