We report an "ionic liquid state" substance composed of superoxide radical anions (O-2(-)) and crownether-coordinated potassium cations. A binary mixture of 18-crown-6-ether (18C6) and potassium superoxide (KO2) becomes liquid above ca. 38 degrees C. The coordination of 18C6 to K+ and the presence of O-2(-) were confirmed in the liquid state by Raman spectroscopy measurements. Below 38 degrees C, however, the 18C6-KO2 mixture does not form a complex but undergoes phase separation to parent phases of solid 18C6 and KO2, because the ion-dipole interaction between K+ and 18C6 is overwhelmed by the ion-ion interaction between K+ and O-2(-). In other words, the pair of O-2(-) and 18C6-coordinated K+ only appears above the melting point of pure 18C6, to form an "ionic liquid state". Among radical-containing ionic liquids, the liquid 18C6-KO2 mixture has high fluidity and comparable conductivity. The findings show stabilization of superoxide radical anions at liquid state with high concentration, which could be used for many radical reactions. (C) The Author(s) 2017. Published by ECS. All rights reserved.