Samples of 1/6Ba(5)Nb(4)O(15).5/6BaNb(2)O(6) along with the pure end members, Ba5Nb4O15 and BaNb2O6, were sintered under low oxygen partial pressure. The degradation mechanisms of dielectric loss in this reducing atmosphere have been studied. We found that the degradation occurred primarily due to the formation of oxygen vacancies caused by the reduction of Nb5+. This was determined by measuring the electrical conductivity, and through X-ray photoelectron spectroscopy. More importantly, the dielectric loss of 1/6Ba(5)Nb(4)O(15).5/ 6BaNb(2)O(6) samples with higher temperature stability was further decreased on sintering in a reducing atmosphere. This observation has been explained by considering the increased porosity and formation of a reduced second phase, Ba0.65NbO3.