The emission yields of SO 2 at 2.0 Torr pressure were measured in the presence of foreign gases at 25° C with incident radiation at 3130 Å. Experiments were performed without and with 0.33 Torr of NO present in order to measure the total and singlet relative emission efficiencies. The foreign gases studied were NO, CO 2, CO, N 2, O 2, N 2O, H 2O, OCS, and thiophene. The main purpose of the study was to extend the quenching gas pressure range beyond that used previously, to see if deviations occurred from the Stern-Volmer quenching plots. Such deviations did occur for the emitting singlet state, 1SO 2, but not for the emitting triplet state, 3SO 2. It is concluded that 1SO 2 is neither the state initially produced when the radiation is absorbed, nor the exclusive precursor state to 3SO 2 formation. Relative quenching constants obtained for the singlet and triplet emitting states agree well with previous work. There is no excess 3SO 2 produced over that predicted from Stern-Volmer quenching.