Maricite, NaFe(II)PO 4 , and sodium iron(III) hydroxy phosphate (“SIHP"), Na 3 Fe(III)(PO 4 ) 2 · (Na 4/3 H 2/3 O), 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 (NaFePO 4 , cr)/(J·K −1 ·mol −1 ) = 107.0 + 8.991 · 10 −2 · (T/K) − 9.371 · 10 5 · (T/K) −2 , (295 K≤T≤ 470 K);C p,m o { Na 3 Fe(PO 4 ) 2 · (Na 4/3 H 2/3 O), cr }/(J·K −1 ·mol −1 ) = 171.0 +0.3272 · (T/K) + 1.538 · 10 6 · (T/K) −2 , (320 K≤T≤ 700 K);Δ f H m o (NaFePO 4 , cr, 298.15K)=−(1571.8 ± 3.0)kJ · mol −1 ; and Δ f H m o {Na 3 Fe(PO 4 ) 2 ·(Na 4/3 H 2/3 O), cr, 298.15K}=−(3830.8 ± 7.0)kJ · mol −1 .