The microjet electrode (MJE)-a new hydrodynamic ultramicroelectrode-is described. In the MJE, a jet of solution is fired at high velocities (up to 25 m s(-1)) through a fine nozzle positioned directly over a disk ultramicroelectrode (UME) with the aid of micropositioners. The electrode is shown to be characterized by well-defined, variable, and high mass-transfer rates under steady-state voltammetric conditions (up to 2 orders of magnitude greater than the conventional diffusion rate) and thus has considerable promise for the study of fast electrode kinetics.