Abstract: A process is described for the preparation of concentrated liquid formulations, stable on storage, of anionic fluorescent brighteners, which process consists in passing a crude (for example obtained from synthesis) aqueous solution or dispersion of a fluorescent brightener containing sulfo groups, in particular a stilbene fluorescent brightener belonging to the group comprising bistriazinylaminostilbenedisulfonic acids, bis-styrylbiphenyls, bis-styrylbenzenes and bis-triazolylstilbenedisulfonic acids, is passed through a semipermeable membrane which contains ionic groups and has a pore diameter of 1-500 .ANG. in order to remove salts and by-products from the synthesis having molecular weights below 500 and in order to remove part of the water. The resulting concentrated preparation can, if desired, be concentrated further and/or treated with formulation assistants and, if desired, further additives.