Orbital symmetry considerations suggest that cheletropic fragmentation of 2, 5-diphenyl-3,4-diazacyclopentadienone-3, 4-dioxide (5) along the thermally allowed pathway would yield benzonitrile oxide and carbon monoxide as primary products. However, experiments show that benzonitrile and carbon dioxide are the major products and solid phase and dilute solution pyrolyses of 5 in complete agreement with its mass spectral fragmentation.