Process for the production of monomeric formaldehyde with a high degree of purity

Formaldehyde of a high degree of purity is made by the thermal decomposition of hemi-formals of poly-valent alcohols in an apparatus having aluminium or austenitic stainless steel surfaces which come into contact with the product and which are treated with fluorine and/or hydrogen fluoride or its aqueous solutions prior to the decomposition of the hemi-formals. It is preferred to treat the reactor surfaces with hydrogen fluoride and phosphoric or nitric acid or with mixtures of hydrogen fluoride and phosphoric or nitric acid. Small quantities, e.g. 0.01-2% by weight of hemi-formal, of hydrofluoric acid may be added to the hemi-formal prior to decomposition. Examples describe the decomposition of ethylene glycol hemi-formal and diethylene glycol hemi-formal.