Peroxides are compounds of structure ROOR in which R may be any organic group. In inorganic chemistry, salts of the anion O2-2. They are strong oxidizing agents.

highly oxidized compounds like hydrogen peroxide (H-O-O-H), which not only oxidize lipids directly but in so doing create free radicals which spread in a chain reaction until stopped (quenched) by enzymes like peroxidases, catalases, and superoxide dismutase or by antioxidants like vitamin E and BHT.

The intermediate compounds formed during the oxidation of lipids which may react further to form the compounds that can cause rancidity.