Highly conductive inorganic-organic hybrid crystals were constructed by using polyoxovanadate cluster and ionic-liquid cation. Two types of hybrid crystals containing alkaline earth metal (divalent) cations were obtained. Conductivities under a humidified condition reached 3.3×10–3 S cm–1 at 353 K (80 °C). More information can be found in the Research Article by Takeru Ito and co-workers.