Mercury(II) hydroxide or mercuric hydroxide is the metal hydroxide with the chemical formula Hg(OH)<sub>2</sub>. The compound has not been isolated in pure form, although it has been the subject of several studies. Attempts to isolate Hg(OH)<sub>2</sub> yield yellow solid HgO.
The solid has produced it by irradiating a frozen mixture of mercury, oxygen and hydrogen. The mixture had been produced by evaporating mercury atoms at 50 ðC into a gas consisting of neon, argon or deuterium (in separate experiments) plus 2 to 8% hydrogen and 0.2 to 2.0% oxygen. The mixture was then condensed at 5 kelvins onto a caesium iodide window, through which it could be irradiated.