The effect of microreactor geometry on performance was studied. In particular, the performance of posted and channel microreactors was compared for a simple test reaction 2NH3 → N2 + 3H2. Geometry played an important role in performance. For example, at 550°C and a residence time of 0.2 sec, a posted reactor gave about 50% conversion while a straight channel reactor gave 98% conversion. Experimentally, the conversion in the posted reactor was as expected for a CSTR, while the conversion in the channel reactor was as expected for a PFR. Flow visualization using smoke to image the flow showed little backmixing in the channel reactor, while the posted reactor showed considerable mixing. RTD measurements showed a narrow residence time distribution for the channel reactor. In contrast, the posted reactor showed a broad residence time distribution with many fluctuations, suggesting that the presence of posts can induce considerable backmixing in a microreactor, even though the flow is laminar. This is an abstract of a paper presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 1/04/2005).