The heat capacity of alpha-Na2NpO4 has been measured for the first time in the temperature range 2.1-301 K using a Quantum Design PPMS (Physical Property Measurement System) calorimeter. The heat capacity and entropy values at 298.15 K have been derived as C-p,m(o) (alpha-Na2NpO4, cr, 298.15 K) = (151.9 +/- 4.6) J K-1 mol(-1) and S-m(o) (alpha-Na2NpO4, cr, 298.15K) = (178.3 +/- 4.3) J K-1 mol(-1), respectively. The standard entropy of alpha-Na2NpO4 follows the trend shown by the isostructural compound alpha-Na2UO4 and the alkaline earth uranates MgUO4, SrUO4, BaUO4, and Cs2UO4. When combined with the entropies of the constituent elements, these data yield a standard entropy of formation at 298.15 K as Delta S-f(m)o (alpha-Na2NpO4, Cr, 298.15 K)=-(385.1 +/- 4.3) J K-1 mol(-1). Using the enthalpy of formation tabulated in the literature, the standard Gibbs energy of formation was calculated at 298.15 K as Delta(f)G(m)(o)(alpha-Na2NpO4, cr, 298.15 K)=(-1649.0 +/- 5.8) kJ mol(-1). (C) 2015 The Authors. Published by Elsevier B.V.