The use of the Nb-Nb2O5 electrode as an oxide ion indicator electrode in potentiometric titrations between acids and bases in fused salts was investigated. Two oxide ion acceptors (acids), NaPO3 and Na4P2O7 were titrated with oxide ion donors (bases) in fused KNO3 The reaction was carried out at 625 K and the bases used in these titrations are K2CO3 and Na2O2. The electrode was found to serve as an excellent indicator for acid base potentiometric titrations in fused salts. It serves also as an oxide ion indicator for the titration of mixtures of acids in which chemical transformations are taking place during the course of titration. The experimental results were discussed in terms of the potential arrests and potential shifts upon the addition of the oxide ion donor to the acid.