Erbium(III) selenate is an inorganic compound, with the chemical formula Er<sub>2</sub>(SeO<sub>4</sub>)<sub>3</sub>. It exists as an anhydrate or an octahydrate.
Monoclinic erbium(III) selenate octahydrate can be crystallized from the solution when dissolving erbium oxide in selenic acid:
Erbium(III) selenate octahydrate is first dehydrated by heating to obtain the anhydrous form, and then heated continuously to obtain erbium selenite and finally erbium(III) oxide.
It can crystallize with M<sub>2</sub>SeO<sub>4</sub> in aqueous solution to form double salts, such as K<sub>3</sub>Er(SeO<sub>4</sub>)<sub>3</sub>÷nH<sub>2</sub>O and NH<sub>4</sub>Er(SeO<sub>4</sub>)<sub>2</sub>÷3H<sub>2</sub>O, etc.