A new high copper ion conductivity solid electrolyte Rb4Cu16I7Cl13 has been found in the CuCI-Cul-RbCI system. This compound has a copper ion conductivity of 0.34 (Ω-cm)-1 at 25°C, and activation energies for conduction of 7.0 kJ/mole between 130°C and − 25°C, and 19 kJ/mole between -25°C and -140° C, respectively. Powder x-ray diffraction analysis indicates that the crystal structure of Rb4Cu16I7Cl13 is simple cubic with a = 10.02Å. © 1979, The Electrochemical Society, Inc. All rights reserved.