The electrochemical reduction of 4-nitrobenzylbromide is studied in NN-dimethylformamide solution in the presence and in the absence of intentionally added supporting electrolyte. By conventional voltammetry it is shown that an ECE-type reaction occurs with formation of the dimer 4,4'-dinitrodibenzyl irrespective of the presence of supporting electrolyte. Next, a micro-gap flow cell is characterised and employed for the preparative electro-reduction in the absence of supporting electrolyte. Excellent yields of the dimer are obtained and explained based on a self-propagation mechanism. (c) 2005 Elsevier B.V. All rights reserved.