A process for the production of a fluorescent whitening agent of formula: ##STR1## in which X and Z are as defined herein, comprising A) rearranging a hydrazobenzene compound having the formula: ##STR2## in which Z is as defined herein, to produce in situ a compound having the formula: ##STR3## B) diazotising the compound of formula (3) to produce a compound having the formula: ##STR4## in which Z has its previous significance and G.sub.1 is a counter ion; and C) reacting the compound of formula (4) with 2 moles of a compound having the formula: ##STR5## in which X and n have their previous significance, in the presence of an inorganic or organic palladium salt, or a mixture thereof, as catalyst-precursor, to produce a compound having the formula (1).