A process for the mass coloration of polyesters which comprises the use of a dye of the formula ##STR1## wherein B represents alkylene of 2 to 4 carbon atoms,Y represents cycloalkyl of 5 to 6 carbon atoms, aralkyl of 7 to 14 carbon atoms or a substituted or unsubstituted carbocyclic or heterocyclic aromatic radical,X represents cyanoalkyl of 1 to 5 carbon atoms or --B--OCO--Y, andA represents an aromatic radical of the formula ##STR2## wherein R.sub.1 represents halogen, cyano, trifluoromethyl or alkylsulphonyl of 1 to 4 carbon atoms, and R.sub.2 represents hydrogen or halogen, or represents the groupD--N.dbd.N--D'--in which D and D' represent substituted or unsubstituted phenyl or naphthyl radicals.