On combustion in excess of air, lithium forms mainly the oxide, Li₂O (plus some peroxide Li₂O₂ ), sodium forms the peroxide, Na₂O₂ (and some superoxide NaO₂ ) whilst potassium, rubidium and caesium form the superoxides, MO₂. Under appropriate conditions pure compounds M₂O, M₂O₂ and MO₂ may be prepared. The increasing stability of the peroxide or superoxide, as the size of the metal ion increases, is due to the stabilisation of large anions by larger cations through lattice energy effects.

Potassium superoxide on hydrolysis gives potassium hydroxide,hydrogen peroxide and oxygen.

$2{\mathrm{KO}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{KOH}+{\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$