Chemiluminescence in the 2900-5200 Å region was observed during deposition and during matrix warming of microwave-discharged mixtures of CS2/N2/Ar, CO/N2/Ar, CH4/N2/Ar and OCS/N2/Ar. An intense progression was assigned as the ν1 sequence of the a3Σ+u → X1Σ+g transition of C2N2. With 13C- and 15N-substitution, the vibrational numbering of the progression was assigned. The data were fitted to yield ν00 = 33245 ± 10 cm-1, ωe = 2351 cm-1, and ωeχe = 9.7 cm-1, if only the ν1 mode was considered. A weaker progression was also observed, which was separated from the intense progression by about 1000 cm-1. Dilution of the mixture with Ar caused an intensity decrease in the C2N2 emission and an increase in the N2 Vegard-Kaplan bands. It is proposed that the C2N2 emission results from the CN + CN matrix reaction and not from an energy-transfer mechanism as proposed previously.