Mixtures of fluorescent whitening agents consisting of 51 to 99% by weight of a symmetrically or unsymmetrically substituted first 1,4-bis(cyanostyryl)benzene and 49 to 1% by weight of an unsymmetrically substituted second 1,4-bis(cyanostyryl)benzene that differs from the first, a process for the preparation of said mixtures by condensing terephthalaldehyde with a di(C.sub.1 -C.sub.4) alkylphosphonomethylbenzonitrile to cyano-4-formylstilbene, which is further condensed, either direct or after isolation, with at least one other di(C.sub.1 -C.sub.4)alkylphosphonomethylbenzonitrile, the use of these mixtures for whitening polyester fibres, and compositions containing said mixtures for whitening polyester fibres.