Paraformaldehyde is obtained by a process comprising (a) concentrating an aqueous formaldehyde solution to a content of 75 to 93% formaldehyde, (b) incorporating a polymerization regulator into the solution, (c) solidifying the solution, and (d) curing the product in the presence of the said polymerization regulator, which is selected from (i) basic organic compounds of a pK 6 between 6 and 12, (ii) hightemperature condensation products of formaldehyde and highly basic amines of pK 6 between 2 and 6, (iii) heterocyclic compounds formed from one, two or three hexatomic condensed and hydrogenated rings in which the hetero atom(s) are nitrogen. Examples of these compounds are (i) pyridine, p-toluidine, phenylhydrazine, hydroxylamine, aniline, trimethyldodecylammonium hydroxide (ii) butylamine and formaldehyde condensation products, and (iii) piperazine and 1 ,4,7,9b-tetraazonaphthalene. The particulate paraformaldehyde may be obtained by spraying the concentrated solution. The concentration step may be performed in two stages with an intermediate curing step.
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