A novel type of hydrodynamic ultramicroelectrode (UME) is described, which employs a ring UME coupled to a high speed perpendicular impinging microjet. Two types of ring UME have been fabricated, both based on an optical fibre, coated in a thin metal film, which is then sealed using either epoxy resin or glass. After polishing, a thin ring UME (approximate to 300-1000 nm) is obtained. When employed in the impinging microjet system, both UMEs show an increase in mass-transport-limited current with flow rate, for simple redox processes such as the reduction of Ru(NH3)(6)(3+) or methyl viologen dication in aqueous solution. However, the mass-transport rates observed are significantly lower than predicted by solving the Navier-Stokes and diffusion equations for an idealised coplanar UME. Characterisation of the UMEs with microscopy reveals imperfections on a 10 nm-1 pin length scale which impact mass-transport significantly. When these imperfections are included in the simulations, it is possible to account for the transport-limiting currents observed experimentally. A general implication of the studies in this paper is that even small perturbations in electrode structure can dramatically influence high-speed convective flow across small-scale UMEs, such that thorough geometric characterisation of UMEs employed in fast-flow systems is important. (c) 2007 Elsevier B.V. All rights reserved.