(HNO₂), an unstable, weakly acidic compound that has been prepared only in the form of cold, dilute solutions. It is useful in chemistry in converting amines into diazonium compounds, which are used in making azo dyes. It is usually prepared by acidifying a solution of one of its salts, the nitrites, which are more stable (see nitrite).

Nitrous acid decomposes into nitric oxide, NO, and nitric acid, HNO₃. It may react as either an oxidizing or a reducing agent; that is, its nitrogen atom may either gain or lose electrons in reactions with other substances. Nitrous acid, for example, oxidizes iodide ion to elemental iodine but reduces bromine to bromide ion.