Microreactors are useful for a variety of different purposes, but guidelines regarding optimal reactor geometry in microreactors remain poorly delineated in the literature. The present work uses the decomposition of ammonia to hydrogen to compare the conversions obtained in posted and channel microreactors. The experiments include measurements using a differential recycle reactor to obtain an empirical expression for the reaction rate, conversion measurements for comparison to idealized reactors, and flow visualization to characterize flow patterns. The conversion characteristics of the two geometries differ greatly. In the range of conditions studied, posted reactors yield conversion behavior close to those expected for ideal mixed flow, while channel reactor behavior resembles ideal plug flow. Flow visualization showed only steady laminar-like flow in the channel reactor. In contrast, the flow pattern in the posted reactor showed significant mixing.