The low temperature heat capacities of Na4UO5 and Na4NpO5 have been measured for the first time in the temperature range (1.9 to 292) K using a Quantum Design PPMS (Physical Property Measurement System) calorimeter. The experimental data have been fitted to theoretical functions below 20 K, and to a combination of Debye and Einstein functions above this temperature. The heat capacity and entropy values at T = 298.15 K have been derived as View the MathML sourceCp,mo(Na4UO5,cr,298.15K)=(220.6±6.7) J · K−1 · mol−1, View the MathML sourceSmo(Na4UO5,cr,298.15K)=(247.5±6.2) J · K−1 · mol−1, View the MathML sourceCp,mo(Na4NpO5,cr,298.15K)=(219.0±6.6) J · K−1 · mol−1, and View the MathML sourceSmo(Na4NpO5,cr,298.15K)=(247.5±6.2) J · K−1 · mol−1. When combined with the enthalpies of formation reported in the literature, these data yield standard entropies and Gibbs energies of formation as View the MathML sourceΔfSmo(Na4UO5,cr,298.15K)=-(520.8±6.3) J · K−1 · mol−1, View the MathML sourceΔfSmo(Na4NpO5,cr,298.15K)=-(521.0±6.3) J · K−1 · mol−1, View the MathML sourceΔfGmo(Na4UO5,cr,298.15K)=-(2301.7±2.9) kJ · mol−1 and View the MathML sourceΔfGmo(Na4NpO5,cr,298.15K)=-(2159.7±6.0) kJ · mol−1.