The photoreactivity of the dicyanoacetylene: acetylene complex, trapped in an argon matrix at 10 K, has been studied at gimel > 120 nm using FTIR spectroscopy and complemented by DFT calculations at the B3LYP/6-31 G** level. Two products have been formed during the experiment. In addition to the spectral bands assigned to dicyanodiacetylene, NC6N, a second set of bands has been observed: its carrier is attributed to NC4NC2 ([3-cyanoprop-2-ynylidyne)ammonio]ethynide) based on DFT calculations and isotopic experiments. On the basis of this assignement, this is the first time that this C6N2 isomer has been identified. (c) 2005 Elsevier B.V. All rights reserved.