Trimethylphenylammonium superoxide (1) and tetrabutylammonium superoxide (2) were prepared by ion-exchange reaction in liquid ammonia. Both compounds were structurally characterized by single-crystal X-ray diffraction. The crystal structure of 2 contains solvent ammonia molecules that are hydrogen bonded to the superoxide ion and therefore may influence the bonding properties of the superoxide ion. The crystal structure of 1 does not contain any solvent molecules. Therefore, it represents the best known approximation to the virtually isolated superoxide ion in the solid state to date. The O-O bond length is 1.332(2) A in 1 and 1.312(2) A in 2. Magnetization measurements show that the susceptibilities of both compounds follow an ideal Curie law down to 2 K reflecting an absence of intermolecular exchange effects between the superoxide ions. The effective magnetic moments of both compounds are larger than the spin-only value due to contributions of the orbital momentum in the superoxide ion. The values of the magnetic moment comply well with the g factors obtained from electron paramagnetic resonance spectra. The g tensors themselves reflect the anisotropic environment of the superoxide ions. The Pi(g) energy levels which are degenerate in the free superoxide ion split up in crystal fields of lower than tetragonal symmetry. The energy splitting is estimated from the diagonal elements of the g tensor of 1.