Two kinds of high ionic conductivity solids, and , have been found in the systems , and at ambient temperature. has a silver ion conductivity of and an activation energy for conduction of 3.8 kcal/mole between 20° and 79°C. has a silver ion conductivity of 0.09 (ohm·cm)−1 at 25°C and an activation energy for conduction of 3.3 kcal/mole between 20° and 147°C. The Ag/electrolyte/I2 solid‐state cells using these electrolytes showed open circuit voltages of 684 mV for , and 689 mV for , which agree well with the value of 687 mV calculated from ΔG° of the cell reaction. Transport number measurements using Tubandt's method showed that the transport numbers of silver ions in both new compounds are 1.00 within the experimental error. The phase diagrams of the systems , and have been determined.