Au and Pt microdisk electrodes (6.4, 12.5, and 25 mu m radii) have been used to investigate the influence of a uniform external magnetic field on faradaic reactions. Magnetohydrodynamic flow within a microscopic volume element adjacent to the microdisk surface results in an increase in steady-state limiting voltammetric currents for reduction and oxidation of both neutral and ionic electroactive molecules. The dependence of limiting currents on external field strength suggests the existence of a threshold value of the magnetic force necessary to induce convective flow. Above this threshold value, the voltammetric current is found to increase in proportion both to the external magnetic field strength and to the steady-state current measured in the absence of the field. The results are used to establish an empirical correlation between faradaic currents and the magnetic force acting on the solution within the depletion layer. In low ionic strength solutions, the external magnetic field also reduces the electrostatic driving force for electron-transfer. This magnetic field-induced reaction overpotential is shown to result from a steady-state displacement of charge-balancing counterions at the electrode/solution interface, analogous to that recently observed for a rotating microdisk electrode.