Preparation of optical brighteners

A process for the preparation of an unsymetrically substituted optical brightener compound of the general formula ##STR52## where X and Y independently of one another are hydrogen, fluorine, chlorine, cyano, C.sub.1 -C.sub.10 -alkoxycarbonyl, unsubstituted or substituted carbamyl or sulfamyl, a sulfonic acid aryl ester group, C.sub.1 -C.sub.10 -alkylsulfonyl or phenyl-sulfonyl, and at least one of X and Y is not hydrogen, and of mixtures of such optical brighteners free of p,p'-substituted compounds, which comprises reacting in a first step terephthaladehyde with a phosphonate compound of the general formula IIa ##STR53## or of the general formula IIb ##STR54## where X and Y have the above meanings in the presence of an alkali, in a solvent from which the monocondensation product of terephthaldehyde and the phosphonate compound precipitates to form a first reaction product containing the monocondensation product and thereafter completing the reaction to prepare a compound or a mixture of compounds of formula (I) by reacting the first reaction product in the presence of alkali and said solvent with a different phosphonate compound of formula IIa or IIb, with the proviso that at least one compound of formula IIa and a compound of formula IIb are reacted successively in the process.