This paper describes ionic conductivity and phase transition in the Li-substituted NASICONs LiM2(PO4)3 (M = Ti, Zr, Hf) and in the systems LiM2(PO4)3-Li2O, LiZr2(PO4)3-Li3PO4 and LiZr2-xMx(PO4)3. The results demonstrate that the conductivity enhancement in various LiM2(PO4)3-lithium salt systems is associated with the formation of solid solutions based on the substitution M4+ reversible 4Li+.