A dimer of HCN, N-cyanoformimine or CH₂NCN, hitherto only postulated as the repetitive unit of a polymer has been produced by pyrolysis (1200-1300 K) of dimethylcyanamide at low pressure (≈30 mtorr) and identified by simultaneous recording of its microwave spectrum (18.6-40.0 GHz). Later, the same dimer was detected in a sample of liquid HCN stored for 1-2 years at 277 K under exclusion of light. The new dimer is, therefore, stable under widely different conditions of pressure and temperature. The possibility of assigning the observed microwave spectrum to iminoacetonitrile or to cyclic azapropenylidenimine is discussed.