Flow-driven instabilites in microdevices, consisting of small features (posts), are studied using 2-D computational fluid dynamics simulations as a means of enhanching mixing at the microscale. It is found that oscillatory flow developed above a critical Reynolds number in the wake of a post is weakly affected by volumetric and surface (catalytic) reactions. The observed reactivity, on the other hand, is strongly affected by flow-driven instabilities. Based on these results, a novel post micromixer that capitalizes on this oscillatory flow while exhibiting a low pressure drop is proposed. Finally, different feature configurations are investigated to gain insights into the mixing performance and to develop design guidelines. (c) 2005 American Institute of Chemical Engineers.