Triplet carbene methylpentadiynylidene, MeC5H (1), was investigated in cryogenic matrices by IR, UV/vis, and EPR spectroscopy. Broadband irradiation (lambda > 497 nm) of the isomeric diazo compounds, 1-diazo-hexa-2,4-diyne (2) or 2-diazo-hexa-3,5-diyne (3), generates triplet carbene 1. EPR spectra yield zero-field splitting parameters (vertical bar D/hc vertical bar = 0.62 cm(-1), vertical bar E/hc vertical bar < 0.0006 cm-1), which are typical for a triplet carbene with axial symmetry. The electronic spectrum of triplet 1 is characterized by a weak absorption in the near-UV and visible region (350-430 nm) with vibronic progressions corresponding to excitations of the acetylenic stretching and the terminal C C-H bending modes. Chemical trapping of triplet 1 in an O-2-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Upon irradiation at A > 399 nm, triplet 1 undergoes photochemical 1,2-hydrogen migration to form hex-1-ene-3,5-diyne (6).