AbstractSynthesis of 2‐(hetaryl or aryl)‐azo thiophene derivatives (IVa–IVg) was achieved by diazotisation of ethyl 2‐amino‐3‐cyano‐5‐carbethoxy thiophene‐4‐acetic acid ester (I) using nitrosyl sulphuric acid, coupling with suitable heterocyclic hydroxy and N, N‐dialkyl‐substituted aryl amines, which resulted in thiophene azo dyes. The key intermediate (I), used as the diazonium component was synthesised in one pot and in excellent yield from diethyl acetone dicarboxylate, sulphur and malononitrile, using diethylamine as a catalyst, following the Gewald Synthesis. The spectral properties of these dyes were studied. The dyes when applied as disperse dyes on polyester fibres