Phase relations and thermodynamic properties of sodium fulleride (Na_xC₆₀)have been measured up to x=6 by the electromotive force (EMF) method. Na beta -alumina was used as the solid electrolyte and Na was added to the cell containing a fixed amount of fullerene by coulometric titration. The phase diagram is characterized by alternate single-phase regions (EMF varying with composition) and two-phase regions (EMF plateaus). At 599 K, the Gibbs energies of formation of Na₃C₆₀ and Na₆C₆₀ are 113.2 and 94.7 kJ/mole of Na, respectively. A model is proposed for the Na activity (or EMF) as a function of composition based on nearest-neighbour interactions, orientational disorder and a random distribution of Na ions. The ionic size and the hindrance of rotation of C₆₀ accounts for the miscibility gap in alkali fullerides in this model.