A crystallization method for 5-amidinium tetrazolide (1) was developed. Crystals of the pure zwitterion 1 enabled the unambiguous assignment of the structure as an inner-salt rather than the amidine tetrazole postulated previously. The solid-state structure of 1 consists of two sets of hydrogen bonds that form a tightly networked two-dimensional sheet. The reaction of amidinium tetrazolide 1 with SOCl2 in the presence of 2.0 equiv. of Et3N produces 3-amino-4-azido-1,2,5-thiadiazole (2) as the sole product. The required tetrazole ring-opening reactivity suggests that the tetrazole moiety may function as a masked geminal azido-nitrene. The thionyl chloride reaction with the amidinium zwitterion 1 occurs not exclusively at the amidine, but rather in the Bay-region of 1, which comprises both the amidinium group and tetrazolide ring. Thiadiazole 2 has a planar structure with a 2D network of hydrogen bonds between the amino group and nitrogen atoms and an SN interaction between the azido group and sulfur.