Laser-induced fluorescence (¿1Au¿¿1Ag) and phosphorescence (¿3Au¿¿1Ag) as well as absorption and excitation spectra of glyoxal in Ar, N2, and CO matrices have been measured at 12 K. Supplementary infrared absorption spectra have also been taken. Although the dominant band in the absorption and excitation spectra is the 000 transition, it is absent in both the fluorescence and phosphorescence spectra leading to the hypothesis that glyoxal forms an exciplex with its nearest neighbor(s) in both the ¿1Au and ¿3Au states. The equilibrium configuration of the nuclei is distorted in the exciplex producing, after fluorescence or phosphorescence, ground state glyoxal molecules born vibrationally excited while the Franck–Condon factors for the 000 transition vanish. In argon the fundamentals ¿7 and ¿12 are preferentially excited, corresponding to two different isomers of the complex, while in N2 and CO an unambiguous assignment for the preferentially excited modes cannot be given. The rule of mutual exclusion still holds in the IR spectra, in agreement with the formation of an excited rather than a ground state complex. Fluorescence and phosphorescence lifetimes place limits on the quantum yields of the various deexcitation channels and show that localized phonon modes play a dominant role in coupling to the matrix.