The standard solubility constant of TPHPH, Th-2(PO4)(2)(HPO4)center dot H2O was evaluated at 298 K by performing dissolution experiments of a synthetic sample in different nitric acid solutions at a constant ionic strength of 1 mol L-1. The average value of the solubility constant and the associated expanded uncertainty U(log(10)K degrees(s,0) (298 K), 0.95 level of confidence) were evaluated to log(10)K degrees(s,0) (TPHPH, 298 K) = -(71.2 +/- 1.3), while the Delta(f)G degrees (TPHPH, 298 K) value reached -(5200 +/- 23) kJ.mol(-1). From the experiments performed at different temperatures, the standard molar enthalpy as well as the standard molar entropy of the dissolution reaction was determined. The solubility of TPHPH was found to be retrograde over the investigated range of temperatures (298-363) K. The values of the standard enthalpy of formation and the standard molar entropy of TPHPH are equal to: Delta H-f degrees (TPHPH) = -(5640.5 +/- 16.0) kJ.mol(-1) and S-m = (298 +/- 68) J K-1 mol (-1), respectively. (C) 2017 Elsevier Ltd.