The compositional ranges of Bi4V2O11 solid solutions containing rare earth elements: Nd, Gd, Er and Yb have been determined by means of a phase diagram study at solidus temperatures, approximate to 900 degrees C. The locus of the solid solutions indicates that ran earth elements replace V in the crystal structure and the new family of oxide ion conductors has the overall formula Bi4+yV2-x-yMxO11-y-x. DTA studies confirmed the identity of the various polymorphs formed. AC impedance studies show that Nd-, Gd-, Er- and Yb-doped materials are oxide ion conductors; however, they are electrically inhomogeneous. The best conductors, irrespective of the dopants, have compositions with y = 0 and x = 0.2 with conductivity values of up to 2 x 10(-4) Ohm(-1) cm(-1) at 300 degrees C.