[1] This yellow-orange solid is a salt of the superoxide anion.

NaO2 is prepared by treating sodium peroxide with oxygen at high pressures:[2] It can also be prepared by careful oxygenation of a solution of sodium in cryogenic liquid ammonia: Although the existence of a sodium oxide higher than peroxide was speculated since 19th century, it was not until 1948 when American chemists were able to definitely synthesize it by the latter method.

[3] It is also produced, along with sodium peroxide, when sodium is stored under inappropriate conditions (e.g. in dirty or partially halogenated solvents)[citation needed].

The product is paramagnetic, as expected for a salt of the O−2 anion.

It hydrolyses readily to give a mixture of sodium hydroxide, oxygen and hydrogen peroxide.