Reactions of thermally or photochemically generated SO4 •-with other species are generally very rapid and have been traditionally investigated using spectroscopic methods. The kinetics of rapid homogeneous reactions that follow an initial electron-transfer step, e.g., the oxidation of Ru(NH3)6 2+ by S2O8 2-(eq. 2), can be investigated using a variety of electrochemical methods, such as fast-scan CV (FSCV) and scanning electrochemical microscopy (SECM). Analysis of data obtained from time-dependent methods, e.g. FSCV, is complicated due to large contributions to the current from capacitive charging and adsorbed species. As a result, SECM analysis, which is generally based on steady-state current-potential (i-E) measurements, has been increasingly employed to measure the kinetics of fast electrochemical reactions. For instance, Bard and coworkers investigated the rate constants of elementary steps involved in the electrooxidation of oxalate and measured the lifetime of transient species CO2 The principles of SECM instrumentation and the electrochemical measurements necessary for kinetic investigations are described in detail elsewhere. Briefly, SECM comprises two electrodes, oriented with their active surfaces parallel to each other. Frequently, a disk-shaped metal tip (radius < 10 µm), referred to as an ultramicroelectrode (UME), is used in conjunction with a larger substrate electrode. The electrode potentials of the tip and the substrate are independently controlled, while the distance between the two electrodes (d) is actively controlled using piezoelectric positioners, and can be varied from large distances, where the electrodes are non-interacting, to very small separations, where interactions occur by diffusion and reactions of redox molecules electrogenerated at the two electrodes. The resulting tip current (it) vs. d plot is referred to as an approach curve and can provide direct experimental insights into the mechanism and kinetics of chemical reactions coupled to an electrode reaction.