A new high ionic conductivity solid Equation or symbol description not availableAg6I4WO4 has been found in the system Equation or symbol description not availableAgIAg2WO4 at ambient temperature. This compound has a silver ion conductivity of 0.047 (ohm · cm)−1 at 25°C and an activation energy for conduction of 3.6 kcal/mole between 20° and 293°C. The transport number measurement using Tubandt's method shows that the transport number of silver ions in this compound is 1.00 within the experimental error. A Equation or symbol description not availableAg/Ag6I4WO4/I2 solid‐electrolyte cell shows an open‐circuit voltage of 687 mV which agrees well with the value of 686.7 mV calculated from Equation or symbol description not availableδG° of the cell reaction at 21°C. Characteristics of the solid‐state cells using Equation or symbol description not availableAg6I4WO4 as the electrolyte have been investigated. The phase diagram of the system Equation or symbol description not availableAgIAg2WO4 has been determined also.