The results of a numerical study of the fundamental interactions of engineering design and micromixing on conversion in packed microchannels are presented. Previously, channel-based microreactors made of molded silicon plastic were designed, fabricated, and experimentally tested. These reactors have enzymes immobilized on the channel walls by various methods including layer-by-layer nano self-assembly techniques. They also contain molded packing features to add reactive surface area and to redistribute the fluid. An arbitrary but intuitively sensible packing arrangement was initially chosen and used in experimental studies. The current computer simulation study was undertaken to understand how static laminar mixing affects the conversion efficiency The reactors previously used experimentally have been simulated using CFD-ACE+ multiphysics software (ESI CFD Inc., Huntsville, AL). It is found that packing significantly increases conversion when compared with empty channels over the entire flow rate range of the study (0.25 < Re < 62.5). The boost in conversion has an optimal point near Re = 20 for the particular geometry examined.