Micropost-filled reactors are commonly found in many micro-total analysis system applications because of their large surface area for the surrounding volume. Design rules for micropost-filled reactors are presented here to optimize the performance of a micro-preconcentrator, which is a component of a micro-gas chromatography system. A key figure of merit for the performance of the micropost-filled preconcentrator is to minimize the pressure drop while maximizing the surface-area-to-volume ratio for a given overall channel geometry. Several independent models from the literature are used to predict the flow resistance across the micropost-filled channels for low Reynolds number flows. The pressure drop can be expressed solely as a function of a couple of design parameters: β = a/s, the ratio of the radius of each post to the half-spacing between two adjacent posts, and N, the number of microposts in a row. Pressure drop measurements are performed to experimentally corroborate the flow resistance models and the optimization scheme using the figure of merit. As the number of microposts for a given β increases in a given channel size, a greater surface-area-to-volume ratio will result for a fixed pressure drop. Therefore, increasing the arrays of posts with smaller diameters and spacing will optimize the microreactor for larger surface area for a given flow resistance, at least until Knudsen flow begins to dominate.