Protonated hydrogen cyanide (HCNH⁺) was first observed and reported by Ziurys & Turner in 1986. Three rotational transition lines were observed in Sgr B2 with the NRAO 12 m telescope and the Texas Millimeter Wave Observatory. A subsequent detection in TMC-1 was reported by Schilke et al.

HCNH⁺ can undergo Dissociative recombination, the addition of an electron followed by dissociation into one or more products. The neutral HCNH intermediate is formed in a very high excited state and can break the CH or NH bond or even both bonds. Experimental results (such as the work of Semaniak et al.) and theoretical calculations (see Taketsugu et al. or Hickman et al.) indicate that HCN and HNC are formed in about the same amounts even though HCN is much more stable than HNC.