The previously reported observations that eq 1 requires initiation by a catalyst or thermal activation is understood by considering the kinetics and thermodynamics of three possible mechanisms for initiating the autocatalytic cycle, as depicted in Scheme 1. The rate constant for S2O8 -bond homolysis at room temperature, eq 4, is estimated to be on the order of 10 -8 s -1 with an activation energy barrier of ~140 kJ/mol. Similarly, the activation energy barrier for S2O8 2-⇄ 2 SO4 •- kf 4 ≈ 10 -8 s -1 (4)