In the environment, arsenic exists predominantly in the +3 and +5 oxidation states. The table below outlines the spoken and written nomenclature that is usually used to describe arsenic's oxidation states.

As(III) is usually considered to be more toxic than As(V). As(III) is usually more prevalent under reduced environmental conditions (e. g., flooded soils). As(V) is usually more prevalent under oxidized conditions (e. g., drained soils). However, both oxidation states of arsenic are often observed together in soils, because of the sometimes slow rates of transformation between oxidation states.

Oxidation state influences many properties of arsenic, including: