The thermodynamic stability of Cu2Se has been measured using the following galvanic cell over the range of 852 to 972 K. Pt, Cu(s), Cu2O(s)//15 YSZ//Cu2Se(s), Cu2O(s), Se(s), C, Pt I The emf of cell I was measured to be (E +/- 0.79) (mV) = 326.58 + 0.153 T(K) By combining available Gibnbs energy data from literature, the Gibbs energy of formation of Cu2Se has been found to be (Delta G(f)(o) (Cu2Se +/- 0.15) (kJ mol(-1)) = -0.0295 T(K) - 63.02 The homogeneity range of beta-Cu2-xSe has been determined by coulometric titration at 900 K on the following cell Pt, Cu2-xSe (s)//15 YSZ//Cu2O(s), Cu (s), Pt II The Cu mole fraction change between the selenium-rich and the copper-rich boundaries was measured as Delta x = 0.012 at 900 K.