Abstract Phase transitions have been observed in mercuric cyanide near 2.5, 8, 19 and 60 kbar. The first four phases have been characterized by Raman spectroscopy. Phase II is closely related to the molecular parent phase I. The coordination at mercury appears to increase to four on entering phase III and phase IV has a spectrum consistent with the cubic structure related to anti-cuprite adopted at s.t.p. by Cd(CN) 2. Above 60 kbar deep brown Hg(CH) 2 V is formed irreversibly: it contains C = N rather than C≡N bonds.