Abstract Using a combination of mass spectrometric techniques, it is shown that 5-amino-4- cyanoisoxazole molecular ions (1+), lose isocyanic acid HN [DOUBLE BOND] O, not fulminic acid, HC [TRIPLE BOND] N [RIGHTWARDS ARROW] O. Metastable ion fragmentations (unimolecular and collision induced) and deuterium-labelling experiments are in agreement with the formation of a cumulenic structure, HN [DOUBLE BOND] C [DOUBLE BOND] C [ ...