Thermal reactions of 1-(thiophene-2-yl)-1H-tetrazoles are reported for the first time. The solution thermolysis of 1-(thiophene-2-yl)-1H-tetrazoles under conventional heating and microwave irradiation afforded the unexpected new thieno[2,3-d]pyrimidines incorporating two thiophene rings, which were obtained as major or only product. The synthesis of these heterocycles was rationalized considering the initial nitrogen elimination to generate imidoylnitrene followed by rearrangement to the corresponding carbodiimide and subsequent cyclization triggered by the nucleophilic attack of the in situ generated 2-aminothiophene. The flash vacuum pyrolysis and solution thermolysis of 1-(thiophene-2-yl)-1H-tetra-zoles also gave thieno[2,3-d]imidazoles via intramolecular formal insertion into a CeC bond of the corresponding imidoylnitrene intermediate.