Compounds of the three large cations tetramethylammonium, tetramethylphosphonium, and tetramethylarsonium with the superoxide radical anion were synthesized by either metathesis or ion exchange in liquid ammonia. They were obtained from concentrated solutions as ammoniates in the form of long needle-shaped single crystals. [N(CH3)4]-(O2)3NH3 crystallizes in the monoclinic crystal system, whereas the two compounds [E(CH3)4](O2)2NH3 (E = P, As) are isostructural and belong to the orthorhombic crystal system. The cation-anion packing in all three crystal structures is related to the sodium chloride structure. All structures contain hydrogen bonds between the ammonia molecules and between ammonia and the superoxide. The solvent of crystallization was easily released from the crystals upon complete removal of the solvent from the reaction vessel, leading to polycrystalline samples. The Raman spectra of all three solvent-free compounds show the symmetric stretching mode of the superoxide ion at about 1123 cm(-1). The desolvated [N(CH3)4](O2) was investigated by powder X-ray diffraction, and the crystal structure was solved by ab initio simulated annealing methods by using rigid-body models of the constituent molecular ions. The superoxide ion shows rotational disorder. The magnetic susceptibility of tetramethylammonium superoxide follows the Curie-Weiss law with a high-temperature effective magnetic moment of 1.66(3) muB and a paramagnetic Curie temperature of theta = -13(6) K. Complementary electron paramagnetic resonance spectroscopy revealed that the average g factor is temperature-dependent. It decreased from 2.15 at 10 K to 1.66 at 100 K, possibly due to the onset of rotational motion of the superoxide ion and in accordance with the lower-than-expected effective magnetic moment.