Microelectrode steady-state voltammetry is used to monitor on-line the progress of preparative electrolyses. This allows to follow educt and product concentrations as a function of the advance of the electrolysis, end-point detection, calculation of the number of electrons transferred per molecule, and determination of diffusion coefficient ratios for redox couples. The technique is demonstrated by numerical simulation, experiments with mixtures of redox partners, electrolyses in macrocells, and miniaturized electrolyses in vials or the wells of microtiter plates. (C) 2005 Elsevier Ltd. All rights reserved.