A calorimetric and thermodynamic investigation of three alkali-metal uranyl tungstates of general formula A(2)[(UO2)(2)(WO5)O], with A = K, Rb and Cs, was undertaken. All three phases were synthesized by high-temperature solid-state reaction of a mixture of the respective alkali-metal nitrates and tungsten (VI) and gamma uranium (VI) oxide. The synthetic products were characterized by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of the three phases was determined using HF-solution calorimetry giving Delta H-f(m)degrees(T = 298 K, K-2[(UO2)(2)(WO5)O], cr) = -4185 +/- 8 kJ.rnol(-1), Delta H-f(m)degrees(T = 298 K, Rb-2[(UO2)(2)(WO5)O], cr) = -4185 +/- 10 kJ.mol(-1), and Delta H-f(m)degrees (T = 298 K, Cs-2[(UO2)(2)(WO5) O], cr) = -4198 9 kJ.mol(-1). Their low-temperature molar heat capacity, C-p,(m)degrees, was measured using adiabatic calorimetry from T = 9 to 329 K for K-2[(UO2)(2)(WO5)O], from T = 9 to 321 K for Rb-2[(UO2)(2)(WO5)O] and from T = 6 to 320 K for Cs-2[(UO2)(2)(WO5)O]. The respective molar third law entropy at T = 298.15 K, S-m degrees, was calculated giving 380 +/- 1 J.K-1.mol(-1) for K-2[(UO2)(2)(WO5)O], 406 +/- 1 J.K-1.mol(-1) for Rb-2[(UO2)(2)(WO5)O] and 419 +/- 1 J.K-1.mol(-1) for Cs-2(UO2)(2)(WO5)O]. These new calorimetric results, combined with literature data, have been used to calculate the Gibbs free energy of formation, Delta(f)G(m)degrees, for all the studied phases giving: Delta(f)G(m)degrees (T = 298 K, K-2[(UO2)(2)(WO5)O], cr) = 3914 8 kJ.mol(-1), Delta(f)G(m)degrees (T = 298 K, Rb-2[(UO2)(2)(WO5)O], cr) = -3915 +/- 10 kj.mol(-1) and Delta(f)G(m)degrees (T = 298 K, Cs-2[(UO2)(2)(WO5)O], cr) = 3926 +/- 9 kJ.mol(-1). Smoothed C-p,(m)degrees(T) values between 0 K and 320 K are presented along with the values for S-m degrees and the functions [H-m degrees(T) - H-m degrees(0)] and [G(m)degrees T) - H-m degrees(0)] for all three phases. The phase relations of various solid phases and solution complexes in the K2WO4-UO3-H2O and K2MoO4-K2WO4-UO3-H2O systems with and without CO2 at T = 298 K were calculated using a Gibbs energy minimization approach. The results show that there is no uranium-containing solid phase stable at pH < 2.2 in either CO2-free or CO2-bearing systems. Here, uranium (VI) is bound in solution complexes. Various solid phases are stable from pH 2.2 to pH 10 in the CO2-free system, but at pH > 10 and in the presence of CO2 no solid is present. (C) 2017 Elsevier Ltd.