Formaldehyde is a simple chemical compound made of hydrogen, oxygen and carbon. All life forms – bacteria, plants, fish, animals and humans – naturally produce formaldehyde as part of cell metabolism.

Formaldehyde is perhaps best known for its preservative and anti-bacterial properties, but formaldehyde-based chemistry is used to make a wide range of value-added products. Formaldehyde is one of the most well-studied and well-understood compounds in commerce.

Formaldehyde is more complicated than many simple carbon compounds in that it adopts several diverse forms. These compounds can often be used interchangeably and can be interconverted.

• Molecular formaldehyde. A colorless gas with a characteristic pungent, irritating odor. It is stable at about 150 °C, but polymerizes when condensed to a liquid.
• 1,3,5-Trioxane, with the formula (CH₂O)₃. It is a white solid that dissolves without degradation in organic solvents. It is a trimer of molecular formaldehyde.
• Paraformaldehyde, with the formula HO(CH₂O)_nH. It is a white solid that is insoluble in most solvents.
• Methanediol, with the formula CH₂(OH)₂. This compound also exists in equilibrium with various oligomers (short polymers), depending on the concentration and temperature. A saturated water solution, of about 40% formaldehyde by volume or 37% by mass, is called “100% formalin”. A small amount of stabilizer, such as methanol, is usually added to suppress oxidation and polymerization. A typical commercial grade formalin may contain 10–12% methanol in addition to various metallic impurities.

“Formaldehyde” was first used as a generic trademark in 1893 following a previous trade name, “formalin

Formaldehyde appears to be a useful probe in astrochemistry due to prominence of the 1₁₀←1₁₁ and 2₁₁←2₁₂ K-doublet transitions. It was the first polyatomic organic molecule detected in the interstellar medium.Since its initial detection in 1969, it has been observed in many regions of the galaxy. Because of the widespread interest in interstellar formaldehyde, it has been extensively studied, yielding new extragalactic sources.A proposed mechanism for the formation is the hydrogenation of CO ice:

H + CO → HC

HCO + H → CH₂O