The ESR spectrum of normal and 17O labeled SO2− in aqueous solutions of Na2S2O4 and in γ-irradiated K2S2O5 has been studied. The 33S and 17O hyperfine tensors are very nearly axially symmetric with parallel and perpendicular principal values of 58.0 ± 0.5 and 4 ± 4 G for 33S and 30.0 ± 0.5 and 3 ± 3 G for 17O. The principal values of the g tensor are 2.0057 ± 0.0005, 2.012 ± 0.003, and 2.0019 ± 0.0020 where the smallest component is parallel to the major 33S and 17O hyperfine components. The results are consistent with the unpaired electron occupying a 2b1 π molecular orbital for which semiempirical coefficients are derived.