Emission spectra have been observed downstream in a flowing afterglow of the hollow-cathode discharge of an At and SO2 mixture. A progression in the 400-600 nm wavelength range is identified as being due to the SO (c(1)Sigma(-) --> X(3)Sigma(-)) transition based on experimentally derived spectroscopic data v(00) = 28 500 cm(-1), omega"(e) = 1155.5 cm(-1) and omega"(e)chi"(e) = 6.6 cm(-1), and careful comparison with the corresponding transition for SO in solid matrix environments. Such an assignment is also consistent with earlier experimental investigation of photofragment SO translation energy and previous quantum computations. This is the first emission spectrum observed for SO(c(1)Sigma(-) --> X(3)Sigma(-)) system in gas phase. (C) 2002 Elsevier Science B.V. All rights reserved.