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 C-p,m(0)(Na4UO5, cr,298.15 K) = (220.6 +/- 6.7) J.K(-1.)mol(-1), S-m(0)(Na4UO5, cr,298.15 K) = (247.5 +/- 6.2) J.K-1.mol(-1), C-p,m(0)(Na4NpO5, cr,298.15 K) = (219.0 +/- 6.6) J.K-1.mol(-1), and S-m(0)(Na4NpO5, cr,298.15 K) = (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 Delta S-f(m)0(Na4UO5, cr,298.15 K) = -(520.8 +/- 6.3) J.K-1.mol(-1), Delta S-f(m)0(Na4NpO5, cr,298.15 K) = -(521.0 +/- 6.3) J.K-1.mol(-1), Delta(f)G(m)(0)(Na4UO5, cr,298.15 K) = -(2301.7 +/- 2.9) kJ.mol(-1) and Delta(f)G(m)(0)(Na4NpO5, cr,298.15 K) = -(2159.7 +/- 6.0) kJ.mol(-1). (C) 2015 The Authors. Published by Elsevier Ltd.