The vibronic spectroscopy of ortho-, meta-, and para-diethynylbenzene (oDEB, mDEB, and pDEB) was studied by two-color resonant two photon ionization (R2PI). The symmetry allowed S-0-S-1 origins of oDEB, mDEB, and pDEB were located at 33 515, 33 806, and 34 255 cm(-1), respectively, with the vibronic structure extending about 2000 cm(-1) above the origin in oDEB and mDEB, but more than 3000 cm-1 in pDEB. Major peaks in each spectrum were attributed to vibronically induced bands, indicating strong coupling of the S, state to the S-2 state. Ground-state infrared spectra in the C-H stretch region (3000-3360 cm(-1)) were obtained using resonant ion-dip infrared spectroscopy (RIDIRS). In all three isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance with a combination band composed of the CdropC stretch and two quanta of the CdropC-H bend. This Fermi resonance was detuned in the overtone region of pDEB, which showed a single peak at 6556 cm(-1). Infrared spectra were also recorded in the excited electronic state using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers, the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. In addition to the sharp C-H stretch features, the oDEB S, infrared spectrum showed a broad absorption stretching from 3050 to 3250 cm-1. Selective deuteration of oDEB at the acetylenic hydrogens led to infrared spectra that showed the broad absorption despite the absence of the acetylenic C-H stretch, indicating that the IR absorption is electronic in nature. Characteristics of this second excited state and its potential relevance as a gateway to a photochemical Bergman cyclization are discussed.