Matrix samples of OCS/O2/Ar (1/1/200), OCS/NO2/Ar (1/1/200), and SO2/Ar (1/100) were photolyzed at 10°K. Subsequent slow warming produced previously observed thermoluminescence due to S 2 and SO2. In addition, a new progression was observed between 5200 and 7700 Å, and, for OCS/O2/Ar samples, a broad emission peak at 7816 Å. With 18O2, the bands in the progression displayed isotopic shifts that permit deduction of ν00 = 21 363 cm-1, ω'e = 1097 cm -1, and ωexe' = 7 cm-1. By analogy to the energy levels of O2 and S2, it is possible to assign the progression to the c̃ 1Σ-→ã 1Δ transition of sulfur monoxide SO. This is the first observation of this transition of SO, and it provides the first spectroscopic information about the c̃ 1∑- state. The 7816 Å peak is attributed to emission by sulfur atom 1S→ 1D. The excitation of sulfur atoms may occur through energy transfer; if so, some of this transfer is from the chemically excited SO*2 (ã 3B1) molecule.