The first reported numbering-up tool, a liquid-flow splitting unit, with three dampening tanks and six interdigital separation-layer micro mixers was recently developed [1]. A minimum/maximum deviation of the 2-propanol/water distribution below 5% and a standard deviation below 2% was achieved. In this article, such an optimised liquid-flow splitting unit was for the first time successfully applied in the field of organic synthesis which in turn weakens flow splitting. As a result, the n-butyl acetamide reaction is a sensitive and very selective test for the quality of flow splitting. n-Butyl acetamide was synthesised from the acid chloride in six parallel (numbered-up) micro reactors with an overall yield of 88% and a purity in a range from 88 to 99%. This reaction is strongly accompanied by fast precipitation. Deviations of ideal flow splitting should thus result for this reaction not only in altered reactant concentrations, but can further change the sensitive flow patterns in the separation-layer mixers, thereby leading to fouling. As most relevant result, the average and min/max deviations in the individual product masses (8%) were larger (as to be expected) than the same quantities for liquid-flow distribution using the non-reacting, model solution 2-propanol/water, albeit not totally out of reach of this benchmark. (C) 2004 Elsevier B.V. All rights reserved.