The oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound.

The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic.

Oxidation states are typically represented by integers, which can be positive, negative, or zero. In some cases the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetite (Fe₃O₄).

The highest oxidation state is +8 in the tetroxides of xenon, ruthenium and osmium, while the lowest is −4 for some elements in the carbon group.

The increase in oxidation state of an atom through a chemical reaction is known as an oxidation; a decrease in oxidation state is known as a reduction. Such reactions involve the formal transfer of electrons, a net gain in electrons being a reduction and a net loss of electrons being an oxidation. For pure elements, the oxidation state is zero.

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