A new lithium ionic conductor of the thio-LISICON (LIthium Superlonic CONductor) family was found in the binary Li2S-P2S5 system; the new solid solution with the composition range 0.0 less than or equal to x less than or equal to 0.27 in Li3+5xP1-xS4 was synthesized at 700 degreesC and characterized by X-ray diffraction measurements. Its electrical and electrochemical properties were studied by ac impedance and cyclic voltammetry measurements, respectively. The solid solution member at x = 0.065 in Li3+5xP1-x S-4 showed the highest conductivity value of 1.5 x 10(-4) S cm(-1) at 27 degreesC with negligible electronic conductivity and the activation energy of 22 U mol(-1) which is characteristic of high ionic conduction state. The extra lithium ions in Li3PS4 created by partial substitution of P5+ for Li+ led to the large increase in ionic conductivity. In the solid solution range examined, the minimum conductivity was obtained for the compositions, Li3PS4 (x = 0.0 in Li3+5xP1-xS4) and Li4P0.8S4 (x = 0.2 in Li3+5xP1-xS4); this conductivity behavior is similar to other thio-LISICON family with the general formula, LixM1-yMy'S-4 (M = Si, Ge, and M' = P, Al, Zn, Ga, Sb). Conduction mechanism and the material design concepts are discussed based on the conductlion behavior and the structure considerations. (C) 2004 Elsevier B.V. All rights reserved.