The “U-phase,” a sodium-containing (alumino-ferrite-monosubstituent) AFm phase, has been observed to form in sodium-enriched highly alkaline cementitious systems, for example, of relevance to nuclear waste, and saline industrial brine management. But, minimal information is available of the U-phase's (e.g., solubility or thermodynamic properties) due to its limited stability and its tendency to transform into ettringite or monosulfoaluminate. Herein, the U-phase was systematically synthesized at four temperatures (5, 15, 25, and 50°C) and fully characterized in terms of its thermochemical properties. The average composition of the synthesized U-phase (4CaO·Al2O3·1.85SO3·0.85Na2O·12H2O) deviates slightly from typical disclosures in the literature. The solubility product of the U-phase formation was measured from conditions of oversaturation. The measured thermodynamic data accurately predicted experimental observations of U-phase formation in cementitious environments. In general, it was noted that the U-phase forms and persists (i.e., remains stable) at pH > 13.7 and [Na+] concentrations superior to 1 mol/L.