It has been reported that irradiation of cyanoacetylene and ozone on a water ice surface at 255 nm yields both hydrogen peroxide and cyanoacetylene (NCCHCO), and it has been proposed that this overall process may involve a crucial step where NCCCHO rearranges to NCCHCO.The isomers NCCHCO and NCCCHO have been prepared in this study by one-electron vertical oxidation of [NCCHCO](-.) and [NCCCHO](-.) in collision cells of a VG ZAB 2HF mass spectrometer. -NR+ experiments indicate that singlet NCCHCO is stable for the microsecond duration of the experiment. In contrast, calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/ 6-31G(d) level of theory indicate that singlet NCCCHO does not occupy a minimum on the singlet neutral potential surface, but rearranges to singlet NCCHCO. Some of the singlet NCCHCO neutrals formed in this way are stable, whereas others have sufficient excess energy to effect decomposition to give NCCH and CO. Triplet NCCCHO is stable but, when energized, may (i) rearrange over a barrier of 49.0 kcal mol(-1) to give triplet NCCHCO, which is energized and decomposes to NCCH and CO, and/or (ii) undergo intersystem crossing to yield singlet NCCHCO. It is concluded that, in principle, the rearrangement NCCCHO to NCCHCO could occur in regions of interstellar ice.