Oxidation is any electrochemical process which involves the formal oxidation state of an atom or atoms (within a molecule) being increased by the removal of electrons. E.g. iron(II) can be oxidized to iron(III):

Fe2+ → Fe3+ + e-

Substances or reactions having the capability to oxidize are said to be oxidative, and oxidative substances are called reductants, or reducing agents.

Formerly, oxidation simply meant the reaction of a material with oxygen (hence the name). However, when the term is now used it is normally in the more general sense. Some common forms of oxidation are the tarnishing of silverware and the rusting of iron:

4Fe + 3O2 → 2 Fe2O3.

Another example is the burning of hydrocarbons to produce water, carbon dioxide, some partially oxidized forms and heat energy. Complete oxidation of materials containing carbon produces carbon dioxide, which is linked to global warming because it absorbs certain wavelengths of infrared light.

In organic chemistry, stepwise oxidation of a hydrocarbon produces water and, successively, an alcohol, an aldehyde or a ketone, carboxylic acid, and then a peroxide.

In inorganic chemistry terms, incompletely oxidized carbon takes the form of carbonate, bicarbonate or carbon monoxide.

The opposite of oxidation is reduction. A reaction involving both processes is called a redox reaction.