Transport processes in an upright, concentric, annular, electrochemical reactor filled with RedOx electrolyte solution are studied experimentally and theoretically. The electrodes form the two vertical surfaces of the reactor. The theoretical calculations consist of the solution of the Navier-Stokes and the Nernst-Planck equations accounting for species' diffusion, migration, convection, and electrochemical reactions on the electrodes' surfaces as a function of the difference in electrodes' potentials and the average concentration of the electrolyte. Since the convection is driven by density gradients, the momentum and mass transport equations are coupled. The current transmitted through the electrolyte is significantly enhanced by natural convection. The current is measured as a function of the difference in the electrodes' potentials. To obtain the reaction rate constants, an inverse problem is solved and the reaction rate constants are determined by minimizing the discrepancy between theoretical predictions and experimental observations. As an example, we study the reversible electrochemical reaction Fe+++ + e(-) = Fe++ on platinum electrodes. (c) 2006 Elsevier Ltd. All rights reserved.