A process for the dyeing or printing of a hydrophobic organic fibrous material containing or consisting of polyester, wherein(a) the fibrous material is dyed (.alpha.) from an aqueous dispersion with a dyestuff of the formula (1) indicated below at a pH value between 2 and 9 and at a temperature above 100.degree. C. or at about 100.degree. C. in the presence of a carrier, or (.beta.) by impregnating it with an aqueous dispersion of a dyestuff of the formula (1), said dispersion having a pH value between 2 and 9, and subsequently exposing the fibrous material so treated to a short heat treatment between about 180.degree. and 210.degree. C., or(b) the fibrous material is printed with a dyestuff of the formula (1) by the transfer printing method, the dyestuff used in methods (a) and (b) being of the formula (1) ##STR1## in which X represents alkyl of 1 to 4 carbon atoms, phenyl, lower alkyl-phenyl, lower alkoxy-phenyl, nitro-phenyl, chloro-phenyl or bromo-phenyl, and D represents phenyl, lower alkyl-phenyl, lower alkoxy-phenylene, nitro-phenylene, chloro-phenylene, benzoyl-phenylene, phenyl-phenylene, phenoxy-phenylene, lower alkyl-sulfonyl-phenylene, aminocarbonyl-phenylene, aminosulfonyl-phenyl, ##STR2## chloro-nitro-phenyl, lower alkyl-nitro-phenyl, lower alkoxy-nitro-phenyl, di(cyano)-phenyl, trifluoromethyl-nitro-phenyl or hydroxyalkoxy-nitro-phenyl.