The high-pressure chemistry of iron in the solid state is rich in new phenomena. Optical studies of the effect of pressure on the interelectronic repulsion parameters and the ligand-to-metal charge transfer peaks, as well as Mössbauer resonance measurements of the change of isomer shift with pressure, indicate that with increasing pressure the 3 d orbitals lower in energy vis-à-vis the ligand orbitals. This has important effects on both the oxidation state and spin state of iron. With increasing pressure Fe(III) reduces to Fe(II) in a wide variety of compounds. This process is analyzed in considerable detail. Examples of the reduction of Fe(IV) to Fe(III) and of Fe(VI) to Fe(IV) and Fe(III) are discussed briefly. The ferrocyanides are the classical low-spin iron compounds. The strong bonding is in part due to the back donation of iron t2 g( π) electrons to the empty π∗ orbitals of the cyanide. The increased affinity of iron for electrons at high pressure reduces this back donation and results in a partial conversion to high-spin Fe(II). The effect of pressure on the spin state of ferricyanides reduced at high pressure, and on substituted cyanides, expecially nitroprussides, is also considered. Two cases of special interest, Prussian blue, Fe 4[Fe(CN) 6] 3, and ferrous nitroprusside, Fe[Fe(CN) 5NO], are discussed at some length.