Crystal structure and oxide ion conductivity of a series of Ti and Ti-Cu double substituted Bi4V2O11 compounds, Bi2V(1-x)TixO(11-x)/2 (0.085 less than or equal to x less than or equal to 0.15), and Bi(2)V(0.9)Cu((0.1-x))TixO(5.35+x) (0 less than or equal to x less than or equal to 0.1), were investigated using X-ray powder diffraction and ac impedance spectroscopy in the temperature and frequency range of 100-700degreesC and 10(-2)-10(7) Hz, respectively.Structural phase transitions, alpha --> beta and beta --> gamma, occur as a function of composition in Ti substituted compounds for which the. is evidenced to be stable at room temperature when x exceeds 0.125. For all Ti-Cu double substituted compounds studied, the room temperature phase was identified to be gamma phase.The required amount of Ti for gamma phase stabilization at room temperature was significantly reduced and the conductivity improved when Cu substituted a part of Ti. Therefore, for the Bi2V0.9Cu(0.1-x)TixO5.35+x (0 less than or equal to x less than or equal to 0.075) compounds the ionic conductivity increased and activation energy decreased with decreasing x. At low temperature, the highest ion conductivity was obtained for Bi2V0.9Cu0.1O5.35. At high temperature (T > 500degreesC), a different behavior was observed. The total conductivity increased at first with decreasing x values down to x = 0.05 and then decreased. The maximum conductivity was obtained for Bi2V0.9Cu0.05Ti0.05O5.4, and the activation energy decreased with decreasing x values, such as what happened at low temperature. (C) 2004 Kluwer Academic Publishers.