The high-pressure structures of nickel monosilicide (NiSi) have been investigated to 124 GPa by synchrotron-based X-ray powder diffraction studies of quenched samples from laser-heated diamond anvil cell experiments, and the equations of state of three of these phases have been determined at room temperature. NiSi transforms from the MnP (B31) structure (space group Pnma) to the-FeSi (B20) structure (space group P213) at 12.5 4.5 GPa and 1550 150 K. Upon further compression, the CsCl (B2) structure (space group Pm3m) becomes stable at 46 3 GPa and 1900 150 K. Thus, NiSi will be in the B2 structure throughout the majority of the Earths mantle and its entire core, and will likely form a solid solution with FeSi, which is already known to undergo a B20 B2 transition at high pressure. Data from the quenched (room-temperature) samples of all three phases have been fitted to the third-order Birch-Murnaghan equation of state. For the MnP (B31) structure this yields K 0 = 165 3 GPa with K 0′ fixed at 4 and V 0 fixed at 12.1499 Å3 atom-1 [V 0 from unpublished neutron diffraction measurements on the same batch of starting material; Wood (2011), personal communication]. For the-FeSi (B20) structure, K 0 =161 3 GPa and K 0′ = 5.6 0.2 with V 0 fixed at 11.4289 Å3 atom-1. For the CsCl (B2) structure, K 0 = 200 9 GPa, K 0′ = 4.6 0.1 and V 0 = 11.09 0.05 Å3 atom -1. The ambient volume of NiSi, therefore, decreases by 6% at the first phase transition and then by a further 3% at the transition to the CsCl structure. Traces of additional NiSi structures predicted by Vočadlo, Wood & Dobson [J. Appl. Cryst. (2012), 45, 186-196; part I], and labelled therein as Pbma-I, Pnma-II, and possibly also Pnma-III and P4/nmm, have been detected.