The article describes a numerical and experimental investigation of the voltammetric response observed within small-scale electrochemical reactors, where reagents are directed to a sensor electrode using hydrodynamic focusing. A microelectrochemical reactor was developed, fabricated and tested for electrochemical sensitivity. The studies presented examine the influence of stream velocity, lead-in length and focusing stream ratios on the electrolytic current induced. The numerical models reveal a regime where the finely focused stream of reagent responds in an identical manner to a cell completely full of reagent and also to a regime where diffusional broadening of the reagent stream into the carrier stream becomes significant with a resulting loss in voltammetric sensitivity. In addition at higher focusing ratios, where the reagent occupies a much smaller portion of the channel, full electrochemical depletion of the species is possible at substantially greater throughput rates than is feasible without the focusing arrangement. Experimental studies show qualitative agreement with numerical predictions. (C) 2009 Elsevier B.V. All rights reserved.