To obtain reliable thermodynamic data for Na2S(s), solid-state EMF measurements of the cell Pd(s)vertical bar O-2(g)vertical bar Na2S(s), Na2SO4(S)vertical bar YSZ vertical bar Fe(s), FeO(s)vertical bar O-2(g)(ref)vertical bar Pd(s) were carried out in the temperature range 870 < T/K < 1000 with yttria stabilized zirconia as the solid electrolyte. The measured EMF values were fitted according to the equation E-fit/V (+/- 0.00047) = 0.63650 - 0.00584732(T/K) + 0.00073190(T/K) In (T/K). From the experimental results and the available literature data on Na2SO4(s), the equilibrium constant of formation for Na2S(S) was determined to be 1g K-f(Na2S(s)) (+/- 0.05) = 216.28 - 4750(T/K)(-1) - 28.288781n(T/K). Gibbs energy of formation for Na2S(s) was obtained as Delta(f)G(o)(Na2S(s))/(kJ (.) mol(-1)) ( +/-1.0) = 90.9 - 4.1407(T/K) + 0.5415849(T/K)ln(T/K). By applying third law analysis of the experimental data, the standard enthalpy of formation of Na2S(s) was evaluated to be Delta H-Gamma(o)(Na2S(s), 298.15 K)/ (kJ (.) mol(-1)) ( +/- 1.0) = -369.0. Using the literature data for C-p and the calculated Delta H-f(o), the standard entropy was evaluated to S-o(Na2S(s), 298.15 K)/(J(.)mol(-1.)K(-1)) (+/- 2.0) = 97.0. (c) 2006 Elsevier Ltd. All rights reserved.