Experimental data on the heat capacities, enthalpies of formation from the oxides and elements of the natural pollucites: Cs0.77Na0.14Rb0.04Al0.91 Si2.08O6.0.34H(2)O (Minas Gerais, Brazil) (I) and Cs0.84Na0.11Al0.88Si2.10O6.0.17H(2)O (East Siberia, Russia) (II) have been determined. The heat capacities of the pollucite (II) were measured by differential scanning calorimetry (DSC) method from T = 250 to 810 K. A value of C(p,m)degrees(298.15K) = 172.2JK(-1) mol(-1) was obtained and the resultant C(p,m)degrees(T) equation in the interval T = 298.15-610K was calculated: C(p,m)degrees = 131.37 + 181.97 x 10(-3)T - 11.84 x 10(5)T(-2) JK(-1) mol(-1) (+/-0.36%). The standard enthalpies of formation were determined by a set of thermochemical cycles using decomposition and subsequent dissolution of the dehydration products in molten lead borate at T = 973 K. Values of Delta(f)H(m)degrees (298.15 K) are found to be -3104 +/- 13 kJ mol(-1) (pollucite I) and -3090 +/- 14 kJ mol(-1) (pollucite II). The values of Delta(f)G(m)degrees (298.15 K) were calculated as -2921 kJ mol(-1) (pollucite I) and -2911 kJ mol(-1) (pollucite II). (C) 2003 Elsevier Science B.V. All rights reserved.