Abstract The high‐temperature phase transition in the novel superprotonic conductor material β ‐ Cs 3 ( HSO 4 ) 2 [ H 2–x ( P 1‐x , S x ) O 4 )] ( x ∼ 0.5) at T p = 398 K was carefully examined by using simultaneous thermo‐gravimetric and differential thermal analysis, modulated differential thermal analysis, and impedance spectroscopy. Our results show evidence that at around this temperature the whole enthalpy‐change is due to dehydration processes that take place on the surface of the crystal and promote high protonic conductivity above this temperature. As a consequence, our findings support the hypothesis that the reported superprotonic‐conducting phase of this material above T p probably does not exist. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)