The numerical values on the standard thermodynamic functions of Ag 2Se (naumannite) were determined by the electromotive force (EMF) method in a solid-state galvanic cell with superionic conductor RbAg 4I5 as the solid electrolyte. Ag2Se was synthesized from pure elements in evacuated quartz glass ampoules and examined to be homogenous by SEM and EDS. According to the experimental data on the EMF versus temperature, the analytical equations were obtained for the polymorphic forms of Ag2Se. The temperature-dependent relationships of the Gibbs energy of formation of Ag2Se in its polymorphic forms and the standard thermodynamic functions of compounds within the temperature range of 350-500 K were also evaluated. The temperature of phase transformation from α-Ag2Se to β-Ag2Se is determined experimentally to be 407.7 K and the enthalpy of phase transformation is 6.06 kJ mol - 1.