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 k(c) was too slow to be measured, and only an upper limit was determined : k(c) less than or equal to 10(4) M(-1) s(-1). Arrhenius parameters were determined at low ionic strength over the range 11.8-74.4 degrees C : 2k(a)/epsilon = (4.8 +/- 2.0) x 10(5) exp(-1.7 +/- 1.1 kJ mol(-1)/RT) cm s(-1) and k(b) = (4.7 +/- 0.1) x 10(3) exp(-15.5 +/- 0.6 kJ mol(-1)/RT) M(-1) s(-1), where epsilon is the SO4- absorption coefficient at 454 nm. At 296 K, the values are in good agreement with literature values : 2k(a)/epsilon = (2.5 +/- 0.2) x 10(5) cm s(-1) and k(b) [H2O] = 440 +/- 50 s(-1). Rate constants k(a) and k(b) were found to increase strongly and nonlinearly with increasing ionic strength (added NaClO4) or acidity (added HClO4). Ion-pair formation provides a possible explanation, and a quantitative empirical model is presented for conditions with [Na+] less than or equal to 1.6 M and [H+] less than or equal to 3 M. Using the ion-pair model, estimated ionization equilibrium constants are obtained for the H+SO4- and the Na+SO4- radical ion pairs.