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Sulfate radical anion SO4- was generated by 248 nm laser flash photolysis of K2S2O8 solutions and monitored by time-resolved multipass absorbance at 454 nm. The 320−520 nm absorption spectrum of SO4- was unaffected by up to 2 M added HClO4, under the experimental conditions. Three reactions were investigated: (a) SO4- + SO4- → S2O82-, (b) SO4- + H2O → HSO4- + OH, and (c) SO4- + S2O82- → products. Rate constant kc was too slow to be measured, and only an upper limit was determined: kc ≤ 104 M-1 s-1. Arrhenius parameters were determined at low ionic strength over the range 11.8−74.4 °C: 2ka /e = (4.8 ± 2.0) × 105 exp(−1.7 ± 1.1 kJ mol-1/RT) cm s-1 and kb = (4.7 ± 0.1) × 103 exp(−15.5 ± 0.6 kJ mol-1/RT) M-1 s-1, where e is the SO4- absorption coefficient at 454 nm. At 296 K, the values are in good agreement with literature values: 2ka/e = (2.5 ± 0.2) × 105 cm s-1 and kb[H2O] = 440 ± 50 s-1. Rate constants ka and kb were found to increase strongly and nonlinearly with increasing ionic strength (added NaCl...