The influence of an external magnetic field on the electrochemical reduction of acetophenone (AP) at Pt and Au microdisk electrodes (radii = 0.1, 6.4, 12.5, and 25 mu m) is described. Voltammetric measurements in CH3CN/[n-Bu(4)N]PF6 solutions containing between 3 mM and 8 M AP demonstrate that the mass-transfer-limited reduction of AP at a microdisk electrode may be significantly enhanced or diminished by an external magnetic field, depending on the redox concentration, the electrode radius, and the angular orientation of the microdisk relative to the field. A mechanism for the magnetic field effect is presented that considers the force arising from the divergent radial flux of electrogenerated radical anion (AP(.-)) through a uniform magnetic field. Viscous drag on the field-accelerated ions results in convective fluid flow that alters the rate at which electroactive AP is transported to the surface. Both lateral and cyclotron fluid motion can be established within a microscopic volume element (similar to 30 nL) near the electrode surface depending on the orientation of the magnetic field with respect to the microdisk. The earth’s gravitational field is demonstrated to enhance or diminish the magnetic field-induced convective flow, depending on the relative directions of the magnetic and buoyancy forces at the electrode surface.