Scheme 1 shows a simple conceptual example of using SECM to measure the rate of a bimolecular reaction involving species generated at the tip. Here, the electrochemical oxidation of species R, present in the bulk solution, at the tip electrode yields species O, which undergoes either a second-order reaction, O + X ® P, to form an electro-inactive species P, or diffuses across the tip-substrate gap to be reduced back to R at the substrate. Herein, we report the use of steady-state voltammetry (SSV) at UMEs and SECM for investigating the mechanism and rates of S2O8 2-reduction using the Ru(NH3)6 3+/2+ electrocatalyst system. As discussed above, prior cyclic voltammetry investigations allowed only estimates of the lower limits of the rate constants for S2O8 3•-dissociation (eq. 3) and the bimolecular electrontransfer reaction between SO4 •-with Ru(NH3)6 2+ (eq. 4). We show that SECM approach curves, obtained using a relatively large tip (3.5-µm radius), can be used to measure the electron-transfer rate constant for the reaction between SO4 •-with Ru(NH3)6 2+ , which approaches a diffusion-limited value (~ 10 9 M -1 s -1 ), corresponding to one of the fastest bimolecular reactions ever measured using SECM. The ability to measure such fast reactions with a large tip is a somewhat counter-intuitive finding and, as shown below, results from spatial localization of homogeneous electron-transfer reactions near both the tip and substrate electrodes. Finally, we present a straightforward approach to fabricate carbon ultramicroelectrodes (UME) with moderately small insulator-to-electrode radius ratios (referred to as the RG ratio in SECM analysis).