It is shown how interdigital micromixers can be optimized in a creeping laminar flow regime. This task is accomplished with an analytical model derived in this paper that does not only include the mixing channel, but also the focusing section typical for interdigital micromixers. The SuperFocus mixer, the genotype interdigital micromixers, is used for this optimization process and it is shown where its bottlenecks with respect to residence time and pressure drop lie and how these can be overcome via a design variation. (C) 2004 Elsevier B.V. All rights reserved.