The application of the radial flow microring electrode (RFMRE) in the measurement of fast heterogeneous electron-transfer kinetics is described. In this new hydrodynamic ultramicroelectrode, solution flows from a capillary, positioned close (5-40 mu m) to a planar glass substrate, and then radially over a thin Pt ring electrode (0.1-0.5,mu m thick) deposited around the outer edge of the capillary. The dimensions and geometry of electrodes are characterized with high precision using conducting atomic force microscopy. Because the thin electrode only sees the Velocity profile in its immediate vicinity, the description of mass transport is effectively analogous to that to a rectangular channel electrode, but with the advantage that the mass transport rate can be changed by altering either the volume flow rate or cell height (nozzle/substrate separation). The high steady-state mass-transfer rates attainable with the RFMRE (coefficients in excess of 2 cm s(-1)) are used to investigate rapid heterogeneous electron-transfer kinetics. A treatment for quasi-reversible electron transfer at the RFMRE is developed from earlier theories for the tubular and channel electrodes. Theoretical results are presented in the form of kinetic indicator diagrams, which allow the standard rate constant, k(o), and transfer coefficient to be derived by simple measurement of (i) the separation between the three-quarter and quarter-wave potentials and (ii) the shift in half-wave potential from the formal potential. It is estimated that k(o) values up to 20 cm s(-1) should be discernible through steady-state measurements at the RFMRE. The method is used to investigate the oxidation of (i) Fe(CN)(6)(4-) in aqueous 0.2 mol dm(-3) KCl and (ii) IrCl63- in aqueous 0.2 mol dm(-3) KNO3. For the former system k(o) is 0.36 +/- 0.06 cm s(-1) (transfer coefficient, beta = 0.42 +/- 0.02), while in the latter case k(o) is 1.9 +/- 0.1 cm s(-1) (beta = 0.56 +/- 0.02).