The thermodynamic stability of Li3NbO4 relative to LiNbO3 and Li2O was determined by oxygen potential measurements in the temperature range 1009-1145 K using a novel composite electrolyte made of alpha + beta alumina. The functioning of the composite electrolyte was independently assessed by emf measurements with well established low oxygen potential electrodes such as Nb/NbO, Ta/Ta2O5 and Al/Al2O3.