Interfaces between alkali metal solid state electrolytes and aqueous solutions are often unstable. In particular, the use of β/β″ alumina superionic conductors is generally limited to conditions absent of liquid water due to their well-known sensitivity to water vapor. However, the degradation mechanism upon exposure to aqueous solutions is not well understood. Using impedance spectroscopy, infrared spectroscopy, and chemical analysis, we studied the mechanism of ionic impedance rise for K+-ion-conducting, polycrystalline K-β″ alumina membranes in room temperature aqueous solutions. By using a non-blocking Fe2+/Fe3+ couple in a symmetric aqueous impedance cell with different concentrations of LiOH, NaOH, KOH, CsOH, and KBr, we find that the rate of resistance rise of the membrane is highly dependent on the pH and K+ concentration in the solution. We find the rate decreases from ~50–200 Ω cm2 h−1 in neutral pH solutions to ~0.1–1 Ω cm2 h−1 in alkaline solutions or solutions with high K+ concentrations. Characterization results are consistent with ion exchange of K+ for hydrated protons as the mechanism of resistance rise.