Peroxides and superoxides are strong oxidizers and are important in chemical processes. Hydrogen peroxide, H₂O₂, prepared from metal peroxides, is an important bleach and disinfectant. Peroxides and superoxides form when the metal or metal oxides of groups 1 and 2 react with pure oxygen at elevated temperatures.

Sodium peroxide and peroxides of calcium, strontium, and barium form by heating the corresponding metal or metal oxide in pure oxygen:

2Na_(s) + O_2(g) → ⎯⎯Δ Na₂O_2(s)

2Na₂O_(s) + O_2(g) → ⎯⎯Δ 2Na₂O_2(s)

2SrO_(s) + O_2(g) → ⎯⎯Δ 2SrO_2(s)

The peroxides of potassium, rubidium, and cesium can prepare by heating the metal or its oxide in a carefully controlled amount of oxygen:

2K_(s) + O_2(g) ⟶ K₂O_2(s) (2mol K per molO₂)

With an excess of oxygen, the superoxides KO₂, RbO₂, and CsO₂ form.

For example:

K_(s) + O_2(g) ⟶ KO_2(s) (1mol K per molO₂)

The stability of peroxides and superoxides of the alkali metals increases as the size of the cation increases.