The cubic gamma-Bi3BaO5.5 shows about half an order of magnitude higher ionic conductivity than that of the structurally distorted monoclinic beta-phase. Solid solutions of the monoclinic isomorph Bi(3-x)M(x)BaO(5.5-x/2) (M = Pb, Cd) have been prepared by solid state reactions. The increase of ionic conductivity in these substituted phases is ascribed to the combined effect of decreasing activation energy and increasing preexponential factor.