1-Propionyl-3,6-diphenyl-1,4-dihydro-1,2,4,5-tetrazine was prepared from propionic anhydride and 3,6-diphenyl-1,4(or 1,2)-dihydro-1,2,4,5-tetrazine, and its structure was determined by X-ray diffraction. This reaction yields the title compound rather than 1-propionyl-3,6-diphenyl-1,2-dihydro-1,2,4,5-tetrazine. The central tetrazine ring of the title compound exhibits a boat conformation and is not, therefore, homoaromatic.