"Current thermodynamic data suggest Ba3(AsO 4 )2(C) to be a highly stable phase. However, sufficient evidence is available to question the validity of the current data. In order to determine the solubility (pK ) and standard free energy of formation (AGO f ) of Ba3(AsO4) 2(c), an equilibrium solubility study was conducted. The equilibrium experiments were carried out under three different ionic strength conditions for reaction times up to 45 d. Equilibrium was approached from both under saturation and supersaturation initial conditions. The PKsp for the dissolution of Ba3(AsO4)2(C) was determined to be 21.62. Correspondingly, a AGO f of -3101.2 - 2.0 kJ.mol-1 was calculated. This value is 166.9 kJ.mol more positive than the current literature value. Reevaluation of the solubility data from a previous work suggest the pK sp for the dissolution of BaHAsO4)II2O(c) to be 24.64 and the corresponding AGo f to be -1587.0 * 17 2.5 kJ.mol -1. An activity diagram analysis of BaHAsO4 and BaHAsO 4. H2 0(C) stabilities indicates the latter to be more stable throughout a wide range of Ba 2 (aq) and AsO4 3-(aq) activities and pii values."