Voltammetric and electrochemical impedance spectroscopy (EIS) measurements have been carried out to study the influence of magnetic fields on mass transport toward ultramicroelectrodes (UMEs). Aqueous solutions of the ferri-ferrocyanide electroactive couple were studied at Pt ultramicrodisk electrodes of 10 and 50 mum diam. Results show that a homogeneous magnetic field B parallel to the electrode surface influences the mass-transport regime toward the UME according to the magnetohydrodynamic (MHD) convection theory developed earlier in our laboratory for electrodes of linear size in the range of a few millimeters. We show that the steady-state voltammetric currents measured at the Pt UME are proportional to (BC*4/3)-C-1/3, where C* is the concentration of the electroactive species. The EIS results confirm the voltammetric results and EIS spectra registered at the UMEs can still be explained in terms of the MHD convection theory. (C) 2003 The Electrochemical Society.