A calorimetric and thermodynamic investigation of two alkali-metal uranyl molybdates with general composition A(2)[(UO2)(2)(MoO4)O-2], where A = K and Rb, was performed. Both phases were synthesized by solid-state sintering of a mixture of potassium or rubidium nitrate, molybdenum (VI) oxide and gamma-uranium (VI) oxide at high temperatures. The synthetic products were characterised by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of K-2[(UO2)(2)(MoO4)O-2] was determined using HF-solution calorimetry giving Delta H-f degrees (T = 298 K, K-2[(UO2)(2)(MoO4)O-2], cr) = -(4018 +/- 8) kJ . mol (1). The low-temperature heat capacity, C-p degrees, was measured using adiabatic calorimetry from T = (7 to 335) K for K-2[(UO2)(2)(MoO4)O-2] and from T = (7 to 326) K for Rb-2[(UO2)(2)(MoO4)O-2]. Using these C-p(o) values, the third law entropy at T = 298.15 K, S degrees, is calculated as (374 +/- 1) J center dot K (1) . mol (1) for K-2[(UO2)(2)(MoO4)O-2] and (390 +/- 1) J . K (1) . mol (1) for Rb-2[(UO2)(2)(MoO4)O-2], These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, Delta(f)G degrees, for both phases giving: Delta(f)G(o) (T = 298 K, K-2[(UO2)(2)(MoO4)O-2], cr) = (-3747 +/- 8) kJ . mol (1) and Delta(f)G degrees (T = 298 K, Rb-2[(UO2)(2)(MoO4)], cr) = -3736 +/- 5 kJ . mol (1). Smoothed C-p(o)(T) values between 0 K and 320 K are presented, along with values for S degrees and the functions [H degrees(T) - H degrees(0)] and [G degrees(T) - H degrees(0)], for both phases. The stability behaviour of various solid phases and solution complexes in the (K2MoO4 + UO3 + H2O) system with and without CO2 at T = 298 K was investigated by thermodynamic model calculations using the Gibbs energy minimisation approach. (C) 2015 Elsevier Ltd. All rights reserved.