This article presents the first systematic study of the effect of Relative Humidity (RH) on the water content and hydroxide ion conductivity of quaternary ammonium-based Alkaline Anion-Exchange Membranes (AAEMs). These AAEMs have been developed specifically for application in alkaline membrane fuel cells, where conductivities of >0.01 S cm⁻¹ are mandatory. When fully hydrated, an ETFE-based radiation-grafted AAEM exhibited a hydroxide ion conductivity of 0.030 ± 0.005 S cm⁻¹ at 30 °C without additional incorporation of metal hydroxide salts; this is contrary to the previous wisdom that anion-exchange membranes are very low in ionic conductivity and represents a significant breakthrough for metal-cation-free alkaline ionomers. Desirably, this AAEM also showed increased dimensional stability on full hydration compared to a Nafion®-115 proton-exchange membrane; this dimensional stability is further improved (with no concomitant reduction in ionic conductivity) with a commercial AAEM of similar density but containing additional cross-linking. However, all of the AAEMs evaluated in this study demonstrated unacceptably low conductivities when the humidity of the surrounding static atmospheres was reduced (RH = 33–91%); this highlights the requirement for continued AAEM development for operation in H₂/air fuel cells with low humidity gas supplies. Preliminary investigations indicate that the activation energies for OH⁻ conduction in these quaternary ammonium-based solid polymer electrolytes are typically 2–3 times higher than for H⁺ conduction in acidic Nafion®-115 at all humidities.