Ab initio and density functional theory (DFT) calculations have been used to study the isomerization process in the C-cyanophosphaethyne (NCCP) system. Minima and transition states have been optimized at the B3LYP/6-311G(2d) level, and single-point energies have been determined at the CCSD(T)/6-311G(ld) level. The lowest energy isomer is found to be the N=C-C=P isomer with C-infinity nu symmetry. The optimized geometry, rotational constant, and dipole moment are in good agreement with experiment. The heat of formation (DeltaH degrees (f,298.15K)) of N=C-C=P has been computed to be 87.1 kcal/mol using an isodesmic reaction with G3 theory.