Disclosed are novel azo dyes from 2-amino-5-organothio-1,3,4-thiadiazoles and N-cyanoethyl-m-acylamidoanilines. These dyes are useful for dyeing synthetic hydrophobic fibers and particularly polyester. The dyes have the general formula: ##STR1## wherein R is alkyl, aryl or cycloalkyl; X is hydrogen, or alkyl; R.sub.1 is hydrogen, alkyl or aralkyl; and R.sub.2 is formyl or alkanoyl; and wherein the alkyl, aryl and cycloalkyl moieties of R and R.sub.2 may be substituted with a large variety of groups such as --OH, OOC-alkyl, --COO-alkyl, succinimido, phthalimido, alkoxy, carbamoyl and the like.