Recent work demonstrates that reactive ion beams can be used to grow compound layers on metal and semiconductor surfaces. Ion beams are characterized by easily controlled ion flux and energy, with a narrow energy spread. Also, the ion species are delivered to the sample in a low pressure environment. These advantages allow greater control and simpler analysis of compound formation processes than other techniques do. Ion beam oxidation is reviewed and compared with thermal, plasma and r.f. oxidation and with oxidation by ion implantation. The ion energy range of several electronvolts to hundreds of electronvolts is suitable for depositing the oxidizing species in the metal in an active state, and simultaneous sputtering can produce a self-limiting oxide thickness when the sputtering yield and oxidation rate are in proper balance. The process of ion beam oxidation is also discussed in light of the etching behavior of metals under combined inert gas and oxygen ion bombardment and of the related technique of reactive ion beam sputter deposition using ion bombardment of a growing film. Additional examples are drawn from the use of other reactive ion beam species, including nitrogen and hydrogen. © 1982.