A process for the preparation of compounds of the formula (I) ##STR1## wherein n represents 0-2;M represents H, alkali metal ion or optionally substituted ammonium ion; andX represents anilino, N- or N,N-alkylamino;by reaction of a compound of the formula (IV) ##STR2## with 2 molar equivalents of an amnine of the formula X--H, at a pH of 5-10, optionally in the presence of an acid-trapping agent which differs from X--H, wherein the compound of the formula (IV) is added to an aqueous reaction medium with a temperature of at least 40.degree. C. and the amine of the formula X--H and, optionally, the acid-trapping agent are added to the aqueous reaction medium independently of one another and/or during and/or after the addition of IV. The compounds of the formula I which are outstandingly suitable as optical brighteners.