Disperse azo dyes having a diaminopyrimidine as coupling component. The dyes give brilliant colorations on textile material of synthetic fibers which have excellent fastness properties, including fastness to light and fastness to dry-heat pleating and setting. These disperse azo dyes are identified by the formula: ##STR1## in which D is the radical of a heterocyclic diazo component or a substituted diazo component of the aniline series, one X is NH.sub.2 and the other X is unsubstituted or substituted cycloalkyl, aralkyl or aryl or substituted alkyl.