Approximate transition energies and oscillator strengths are computed with large-scale multi-reference configuration interaction calculations to di3erentiate between linear pentadiynylidene HC5H and its Cs symmetric isomer ethynylcyclopropenylidene HC3C2H by certain fingerprints in their electronic spectra. The first dipole-allowed transition of the HC5H spectrum is obtained at 2.76 eV (3∑u- ← X 3∑g-) in reasonable agreement with experimental measurements of 434 nm (2.86 eV) in neon matrix. The oscillator strength of 3∑u- ← X 3∑g- of linear HC5H is computed with f = 0.007, a value similar to what we obtain for linear HC7H (f = 0.005). In addition a second very strong 3∑u- ← X3∑g- transition is predicted by the calculations around 7.3 eV. The yet-to-be-observed electronic spectrum of HC3C2H is characterized by four strong transitions calculated at 4.79 eV (2 1A″), 6.17 eV (2 1A′), 7.27 eV (3 1A″) and 7.74 eV (4 1A′). The first transition of HC3C2H 1 1A″ ← X 1A′ is calculated to be very weak and at much higher energy (3.64 eV) than the first 3∑u- ← X 3∑g- transition of the linear isomer.