Advanced oxidation processes based on sulfate radicals (SO4·−) are capable of efficiently degrade organic pollutants from ground, surface and wastewaters. However, this degradation may be limited by aqueous natural organic matter (NOM). Here we measured the absolute rate constants of reaction of SO4·− with four types of organic matter: two fulvic acids and two lake organic matter. We used laser flash photolysis technique to monitor the SO4·− decay and the formation of the transients from organic matters. Reaction rate constants comprised between 1530 and 3500 s−1 mgC−1 L were obtained by numerical analysis of differential equations and the weighted average of the extinction coefficient of the generated organic matters radicals between 400 and 800 M−1 cm−1.