an organic compound, the simplest of the aldehydes, used in large amounts in a variety of chemical manufacturing processes. It is produced principally by the vapour-phase oxidation of methanol and is commonly sold as formalin, a 37 percent aqueous solution. Formalin may be dehydrated to trioxane, a crystalline trimer, or to an amorphous polymer, paraformaldehyde, which is a convenient source of gaseous formaldehyde.

Formaldehyde and ammonia yield methenamine, or hexamethylenetetramine, which is used as a urinary antiseptic. Nitration of methenamine gives the explosive cyclonite, or RDX. Formaldehyde and acetaldehyde react in the presence of calcium hydroxide to give pentaerythritol, the tetranitrate of which is the explosive PETN. Large quantities of formaldehyde are used in the manufacture of urea–formaldehyde, phenol–formaldehyde, and acetal resins. The reaction of formaldehyde with proteins leads to its use in the tanning industry and in treating various vegetable proteins to render them fibrous. The reactivity with proteins is also the basis for the use of formaldehyde as a disinfectant, an embalming agent, and a soil sterilant.

Pure formaldehyde is a colourless, flammable gas with a strong pungent odour; it is extremely irritating to the mucous membranes.