Extreme non-Arrhenius dependence of the ionic conductivity in optimized fast ion conducting glasses has been observed. When all the chemical factors controlling the ionic conductivity in glass have been optimized, the conductivity fails to reach the values expected, >0.1 (Ωcm)−1 at 298 K. A new series of glasses zAgI+(1−z) [0.525Ag2S+0.475B2S3:SiS2] have been measured for the first time and are found to exhibit a non-Arrhenius conductivity, the extent of which increases the greater the AgI content. Such behavior is believed to be a new feature of optimized fast ion conducting glasses and will be a critical obstacle to overcome if the conductivity of such systems is to ever reach the values needed for optimum device performance.