5-Amino-(tert-butyl-NNO-azoxy)-1,2,3,4-tetrazine 1,3-dioxide reacts with nitronium tetra-fluoroborate to give 5-amino-6-nitro-1,2,3,4-tetrazine 1,3-dioxide. This compound is of interest as a new energetic material. A plausible reaction mechanism involves electrophilic substitution of the tert-butyl-NNO-azoxy group by the nitro group.