The halogens react with each other to form interhalogen compounds.

The interhalogens of form XY have physical properties intermediate between those of the two parent halogens. The covalent bond between the two atoms has some ionic character, the less electronegative element, X, being oxidised and having a partial positive charge. Most combinations of F, Cl, Br and I are known, but not all are stable.

Some compounds of the general formula XY_n are known, where n is 3, 5 or 7 (X, again, is the less electronegative of the two). ClF₃ is used in the manufacture of uranium hexafluoride. They are all prone to hydrolysis, and ionise to give rise to polyatomic cations and anions.

Bromine monofluoride has not been obtained pure - it dissociates into the trifluoride and free bromine. Similarly, iodine monofluoride is unknown - iodine reacts with fluorine to form a pentafluoride.

Iodine monochloride consists of red transparent crystals which melt at 27.2° to form a choking brownish liquid. It reacts with HCl to form the strong acid HICl₂. The crystal structure of iodine monochloride consists of puckered zig-zag chains, with strong interactions between the chains.

Iodine monobromide is made by direct combination of the elements to form a red liquid which freezes at 42° and boils at 116° to form a partially dissociated vapour.

Chlorine trifluoride, ClF₃ is a colourless gas which condenses to a green liquid, and freezes to a white solid. It is made by reacting chlorine with an excess of fluorine at 250° in a nickel tube. It reacts more violently than flourine, often explosively. The molecule is planar and T-shaped.

Bromine trifluoride is a yellow green liquid which conducts electricity - it ionises to form [BrF₂⁺] + [BrF₄^-]. It reacts with many metals and metal oxides to form similar ionised entities; with some others it forms the metal fluoride plus free bromine and oxygen. It is used in organic chemistry as a fluorinating agent. It has the same molecular shape as chlorine trifluoride.

Iodine trichloride forms lemon yellow crystals which can be melted under pressure to a brown liquid. It can be made from the elements at low temperature, or from iodine pentoxide and hydrogen chloride. It reacts with many metal chlorides to form tetrachloriodides, and hydrolyses in water. The molecule is a planar dimer, with each iodine atom surrounded by 4 chlorine atoms.

Bromine pentafluoride is a colourless fuming liquid, made by reacting bromine trifluoride with fluorine at 200°. It is physically stable, but reacts violently with water and most metals and non-metals.

Iodine pentafluoride is a colourless liquid, made by reacting iodine pentoxide with fluorine, or iodine with silver fluoride. It is highly reactive, even slowly with glass. It reacts with elements, oxides and carbon halides. The molecular has the form of a tetragonal pyramid.

Iodine heptafluoride is a colourless gas. It is made by reacting the pentafluoride with fluorine. IF₇, is chemically inert, having no lone pair of electrons in the valency shell. The molecule is a pentagonal bipyramid.