Mass-selective threshold photoelectron spectroscopy in the gas phase was employed to characterize the dialkynyl triplet carbenes pentadiynylidene (HC5H), methyl-pentadiynylidene (MeC5H), and dimethylpentadiynylidene (MeC5Me). Diazo compounds were employed as precursors to generate the carbenes by flash pyrolysis. The R-1-C-5-R-2 carbon chains were photoionized by vacuum ultraviolet (VUV) synchrotron radiation in photoelectron photoion coincidence (PEPICO) experiments. High-level ab initio computations were carried out to support the interpretation of the experiments. For the unsubstituted pentadiynylidene (R-1 = R-2 = H) the recorded spectrum yields an adiabatic ionization energy (IEad) of 8.36 +/- 0.03 eV. In addition, a second carbene isomer, 3- (didehydrovinylidene)cyclopropene, with a singlet electronic ground state, was identified in the spectrum based on the IEad of 8.60 +/- 0.03 eV and Franck-Condon simulations. We found that multireference computations are required to reliably calculate the IEad for this molecule. CASPT2 computations predicted an IEad = 8.55 eV, while coupled-cluster computations significantly overestimate the IE. The cyclic isomer is most likely formed from another isomer of the precursor present in the sample. Stepwise methyl-substitution of the carbene leads to a reduction of the IE to 7.77 +/- 0.04 eV for methylpentadiynylidene and 7.27 +/- 0.06 eV for dimethylpentadiynylidene. The photoionization and dissociative photoionization of the precursors is investigated as well.