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 the electrodes' potentials and the average concentration of the electrolyte. Since the convection is driven by density gradients, the momentum and mass transport equations are strongly coupled. In spite of the small dimensions (mm-scale) of the reactor, 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. The paper demonstrates that natural convection plays a significant role even when the reactor’s dimensions are on the millimeter scale and that it is possible to predict reaction rate constants while accounting for significant mass transfer effects.