Heat capacity measurements of the crystalline phosphates NaTi(2)(PO(4))(3) and NaHf(2)(PO(4))(3) were performed between (6 and 650) K. Their thermodynamic functions, molar heat capacities C(p,m)(0), enthalpy [H degrees(T) - H degrees(0)], entropy S degrees(T), and Gibbs free energy G degrees(T) - H degrees(0), over the range from T --> 0 to 650 K, were calculated, and the fractal dimension D(fr) was evaluated. Standard entropies of formation at T = 298.15 K were estimated to be [(1174 +/- 2) and (1207 +/- 2)] J.K(-1).mol(-1) for NaTi(2)(PO(4))(3) and NaHf(2)(PO(4))(3), respectively.