A process is described for improving the photochemical stability of dyeings on polyester fibre materials by means of UV absorbers of the formula ##STR1## in which R is lower alkyl, lower alkoxy, halogen or hydroxyl, R.sub.1 and R.sub.2 independently of one another alkyl which is substituted by hydroxyl, lower alkoxy, lower alkylthio, amino, monoalkylamino or dialkylamino, phenyl, phenyl which is substituted by chlorine, lower alkyl or lower alkoxy, or o-hydroxyphenyl, and n is 0, 1 or 2.