Formaldehyde (CH2O) is the only gaseous aldehyde. It is dissolved in water with a saturation of 37 to 40%. This solution is generally referred to as "formaldehyde" or "formaldehyde solution". For this purpose, this concentrated solution is used by diluting it with water or buffer (usually 1/10), then 10% formalin solution, which in fact contains 4% formaldehyde, an ideal concentration for fixing. It is also possible to use paraformaldehyde which is in the form of a powder, this compound being the highly polymerized form of formaldehyde. Paraformaldehyde makes it possible to prepare very pure solutions of formaldehyde and is particularly recommended for electron microscopy.

Formaldehyde is a fast-penetrating, slow-fixing fixative that is best suited for large pieces of tissue. Formaldehyde is a chemical fixative that reacts with side chains of proteins to form hydroxymethyl groups. The effects of formaldehyde are reversible by excess water.