Infrared spectra of UV-photolysed argon matrices doped with cyanoacetylene reveal the presence of HNCCC, a carbene (imine) isomer of the parent molecule. The identification of three stretching bands is based on deuterium and nitrogen-15 substitution experiments and on a B3LYP/6-311++G(**) density functional theory study. The detection of this energetically unfavourable isomer is facilitated by the high absolute intensity of its IR absorption.