The present work investigates in-situ the thermal behaviour of alum-(Rb), RbAl(SO4)2 · 12H2O, by high-temperature X-ray powder diffraction from 303 K to melting, which starts at 359 K and is completed, due to kinetics, at 363 K. The calculated a0 linear thermal expansion coefficient is 10.33(12) x 10-6 K-1 within the investigated thermal range. The k order parameter, which is the measure of the extension of the orientational disorder of the sulphate group, decreases from ca. 0.80 at 303 K to ca. 0.75 just before melting. This behaviour has been shown to depend on the need to keep the bond valence sum of the Rb+ cation, which coexists in both six- and seven-fold coordination, at a reasonable value of ca. 0.82 valence units
