Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound with chemical formula . It is the simplest geminal diol. In aqueous solutions it coexists with oligomers (short polymers). The compound is closely related and convertible to the industrially significant derivatives paraformaldehyde (), formaldehyde (), and 1,3,5-trioxane ().
Methanediol is a product of the hydration of formaldehyde. The equilibrium constant for hydration is estimated to be 10, predominates in dilute (<0.1%) solution. In more concentrated solutions, it oligomerizes to .