Fast ion conductors based on Li1-xM2-xM′xP3O12 (M = Hf, Zr; M′ = Ti,Nb) structures have been synthesized and the electrical transport properties are studied to determine their ionic conductivities. ac conductivity analysis is carried out to understand the characteristic properties of the compounds. Results are discussed in terms of unit cell size, bottleneck between vacant sites and the lithium content. © 1989.