In chemistry, peroxides are a group of compounds with the structure R−O−O−R, where R is any element.^[1][2] The O−O group in a peroxide is called the peroxide group or peroxy group (sometimes called peroxo group or peroxyl group). The nomenclature is somewhat variable,^[3] and the term was introduced by Thomas Thomson in 1804 for an oxide with the greatest quantity of oxygen.^[4]

The most common peroxide is hydrogen peroxide (H₂O₂), colloquially known simply as "peroxide". It is marketed as solutions in water at various concentrations. Many organic peroxides are known as well.

Structure and dimensions of H₂O₂ in gas phase.

In addition to hydrogen peroxide, some other major classes of peroxides are:

• Peroxy acids, the peroxy derivatives of many familiar acids, examples being peroxymonosulfuric acid and peracetic acid, and their salts, one example of which is potassium peroxydisulfate.
• Main group peroxides, compounds with the linkage E−O−O−E (E = main group element).
• Metal peroxides, examples being barium peroxide (BaO₂), sodium peroxide (Na₂O₂) and zinc peroxide (ZnO₂).
• Organic peroxides, compounds with the linkage C−O−O−C or C−O−O−H. One example is tert-butylhydroperoxide.