Maricite, NaFe(II)PO4, and sodium iron(III) hydroxy phosphate ("SIHP"), Na3Fe(III)(PO4)(2) . (Na4/3H2/3O), are recently discovered compounds that play an important role in phosphate-based water treatments for high-pressure steam generators in the electric power industry. Heat capacities of maricite and SIHP have been determined as a function of temperature by differential scanning calorimetry, and enthalpies of solution have been measured at T = 298.15 K in an isothermal calorimeter. These results lead to the following expressions for the standard molar heat capacity and enthalpy of formation of maricite and SIHP: C-p,m(o)(NaFePO4, cr)/(J . K-1 . mol(-1)) = 107.0 + 8.991 . 10(-2) . (T/K) - 9.371 . 10(5) . (T/K)(-2), (295 K less than or equal to T less than or equal to 470 K); C-p,m(o){Na3Fe(PO4)(2).(Na4/3H2/3O), cr}/(J . K-1 . mol(-1)) = 171.0 + 0.3272 . (T/K) + 1.538 . 10(6) . (T/K)(-2), (320 K less than or equal to T less than or equal to 700 K); Delta(f)H(m)(o)(NaFePO4, cr,298.15 K) = -(1571.8+/-3.0)kJ . mol(-1); and Delta(f)H(m)(o){Na3Fe(PO4)2 .(Na4/3H2/3O), cr,298.15 K} = -(3830.8+/-7.0) kJ . mol(-1)