The present invention provides a hydrate of the 4,4′-di-triazinylamino-2,2′-di-sulfostilbene compound having the formula: in which M and M1 independently represent hydrogen, an alkaline-, an alkaline earth metal or ammonium, x is a number within the range of from 1 to 30, and the crystal form of the hydrate (I) being characterised by an X-ray diffraction pattern which is essentially as set out in the accompanying FIGS. 1 to 11; or a mixture containing one or more of the hydrates of the 4,4′-di-triazinylamino-2,2′-di-sulfostilbene compound having the formula (I); processes for the preparation of the new hydrates; and the use of the new hydrates for the preparation of concentrated aqueous formulations of fluorescent whitening agents.