The electronic structure, vibrational spectra, and thermodynamic stability of CCCN and HCCCN have been determined using ab initio molecular orbital theory. The heat of formation for HCCCN was determined using Gaussian-2 theory (G2) and was found to be 91.5+/-2 kcal mol(-1). There is good agreement between the G2 value and that determined using an isodesmic reaction (i.e., one in which both the number and types of bonds are conserved), which yields 90.7+/-2 kcal mol(-1). The heat of formation for the CCCN radical is 178.2+/-2 kcal mol(-1). These data are used to compute the bond dissociation energies in HCCCN, D-0(C-C) and D-0(C-H), which are important in assessing the chemistry in Titan’s atmosphere, as well as in other interstellar media.