It is a red or yellow solid. It is very toxic and corrosive. It reacts violently with some reducing agents, as it is an oxidizing agent. It reacts with acids to make other mercury(II) salts like mercury(II) chloride. It breaks down when heated to make mercury and oxygen.
It can be made by heating mercury and oxygen at 350 °C (662 °F). It can also be made by heating mercury(II) nitrate. This makes the red form. The yellow form is made by reacting a mercury(II) salt with a strong base.