Continuous wave and pulsed electron paramagnetic resonance studies were performed on the superoxide ion, O-2(-), encaged in the nanoporous crystal 12CaO(.)7Al(2)O(3) (C12A7). Hyperfine splitting due to O-17 in O-2(-) was observed in C12A7 heated in 40% O-17-enriched O-2 gas at 550 degreesC, providing solid evidence that encaged O-2(-) derives from gaseous O-2 in the atmosphere via reactions with free oxygen ions in the cage. Angular variations of g values, O-17 hyperfine splitting in the single crystal at 20 K, and Al-27 electron spin-echo envelope modulation pattern for the powder at 4 K clarified that an O-2(-) is encaged in a subnanometer-sized cage of C12A7 and adsorbed on a Ca2+ ion forming a part of the cage wall. Further, two oxygen atoms in the O-2(-) ion are equivalent in the cage, indicating that the O-2(-) ion takes a "side-on" configuration. Then, the O-O bond is perpendicular to the 2-fold rotation axis (C-2 parallel to <100>) across the center of the cage and directed to two oxygen ions forming the cage wall. Variation in the g values with temperature indicates that the O-2(-) may be regarded as solidlike below similar to20 K and above this temperature the rocking motion of the O-O bond about the C2 axis is activated with an increase in temperature, but the anisotropy in the motion remains even at 400 K. This result may be explained by a view that Coulombic interaction of the Ca2+ with the fully occupied pi orbital in the O-2(-) is stronger than that with the semioccupied pi orbital, controlling the dynamics.