A new hydrodynamic microelectrochemical reactor design is presented for the voltammetric sensing of chemical species contained within two immiscible liquid streams flowing within rectangular ducts, in direct contact. This article describes the design, fabrication and experimental characterisation of the device. A microfabricated rectangular duct (of typical dimensions: height 75 μm. width 500 μm and length 3 cm) was constructed using FOTURAN glass and standard photolithographic procedures. Microelectrode sensors were positioned on one internal duct wall with a geometry to permit separate voltammetric monitoring of the two solvent phases. Reagent solutions containing N,N,N′,N′-tetramethyl-1,4-phenylene diamine in 1,2-dichloroethane and hexaamineruthenium(III)chloride in water were pumped through the device under laminar flow conditions. Linear sweep voltammetric measurements were performed separately on the two electrolyte streams and the variation of the transport limited current as a function of volume flow rate through the cell monitored. Under conditions where stable flow was obtained the current flow rate relationship was observed to follow analogous voltammetric behaviour to that observed in macroscopic flow cell devices. © 2002 Elsevier Science B.V. All rights reserved.