Thien-3-ylazo dyes of the formula: ##STR1## where X is cyano, nitro, C.sub.1 -C.sub.6 -alkanoyl, substituted or unsubstituted benzoyl, substituted or unsubstituted phenyl or the radical COOR.sup.1 or CONR.sup.1 R.sup.2 where R.sup.1 and R.sup.2 are identical or different and each is independently of each other substituted or unsubstituted C.sub.1 -C.sub.8 -alkyl, C.sub.5 -C.sub.7 -cycloalkyl, C.sub.3 -C.sub.4 -alkenyl, substituted or unsubstituted phenyl or together with the nitrogen atom linking them a 5- or 6-membered saturated heterocyclic radical, and R.sup.2 may additionally be hydrogen, Y is hydrogen, cyano, C.sub.1 -C.sub.6 -alkanoyl, substituted or unsubstituted benzoyl, C.sub.1 -C.sub.4 -alkylsulfonyl, substituted or unsubstituted phenylsulfonyl, substituted or unsubstituted phenyl or the radical COOR.sup.1 or CONR.sup.1 R.sup.2 where R.sup.1 and R.sup.2 are each as defined above, Z is hydrogen, substituted or unsubstituted C.sub.1 -C.sub.8 -alkyl, substituted or unsubstituted phenyl, substituted or unsubstituted C.sub.1 -C.sub.6 -alkylthio, substituted or unsubstituted phenylthio, substituted or unsubstituted C.sub.1 -C.sub.6 -alkylsulfonyl or phenylsulfonyl or together with the radical Y a fused benzo ring which may be substituted, and K is the radical of a coupling component of the 2,6-diaminopyridine, 6-hydroxypyrid-2-one or 5-aminopyrazole series, are used for dyeing synthetic fiber materials, in particular synthetic polyesters, in yellow to red shades and are notable for good light fastness, fastness to dry heat setting and pleating and for good affinity.