A new Aurivillius phase, (Bi2O2)(NaNb2O6.5), with intrinsic oxygen vacancies has been synthesized. The average structure is very close to that of the parent phase (Bi2O2)(CaNb2O7). The new phase displays a discontinuous change in the Arrhenius plot of the ionic conductivity versus temperature. Differential thermal analysis (DTA) confirmed the presence of a reversible phase transition at 860 degrees C. At 900 degrees C, the conductivity of (Bi2O2)(NaNb2O6.5) is 10(-2)S cm(-1). At 1000 degrees C, the Aurivillius phase slowly decomposes to a mixture of phases with a lower total conductivity.