The theory of collection efficiencies at a double tubular electrode is explored through numerical simulation, including an analysis of the effects of axial diffusion. The theory is compared with analytical results in the high flow rate limit, using the Levich equation and the Matsuda theory for collection efficiency. For lower flow rates the dependence of the collection efficiency on absolute and relative lengths and radii of the tubular electrodes is investigated. (c) 2005 Elsevier B.V. All rights reserved.