Thermal decomposition of ethyl azidoformate in excess of dimethyl terephthalate (DMT) (1) in acetic anhydride as solvent results in the formation of N,N-dicarbethoxy-4,8-dicarbomethoxy-2,6-diazabicyclo[3.3.0]octa-3,7-diene (3) in addition to N-carbethoxy-2,5-dicarbomethoxy-1H-azepine (2) and ethyl N-(2,5-dicarbomethoxyphenyl)carbamate (4). The structure of the diazabicyclooctadiene (3) was established on the basis of spectral, elemental, and X-ray crystallographic analyses.