Measurements of the enthalpy increment, (H(T)THETA - H298.15THETA) for Rb2U(SO4)3 were carried out in the range 370-800 K using a high-temperature Calvet calorimeter. Solid state transitions were observed at 625 +/- 3 K and 763 +/- 2 K, with respective enthalpies’s of transitions of 5.998 and 13.585 kJ mol-1. The H(T)THETA - H298.15THETA values were plotted against temperature and can be given in J mol-1 by H(T)THETA - H298.15THETA +/- 694/(J mol-1) = -1.117 X 10(5) + 368.9T/K + 0.0199T2/K2 (from 370 to 628 K); H(T)THETA - H298.15THETA +/- 405/(J mol-1) = 3.279 X 10(3) + 13.4 T/K + 0.3063T2/K2 (from 630 to 760 K); and H(T)THETA - H298.15THETA +/-433/(J mol-1) = -6.671 X 10(5) + 1140T/K (from 765 to 803 K). The thermodynamic properties of Rb2U(SO4)3(s) were derived using the estimated value of S298.15THETA.