In a recent experimental study of reaction rates measured in a rectangular duct with a reaction of arbitrary order occurring on one wall, the results were analyzed in terms of an infinite parallel plate duct. The subject of this paper is the determination of the requirements necessary to treat a rectangular duct as a flat duct in the fully developed laminar flow regime. To this end, the rectangular duct problem is solved numerically. The results obtained show that mixing-cup concentrations, average wall concentrations, and Nusselt numbers for any aspect ratio γ (between a square duct and parallel plates) may be estimated from the parallel plate results when a reduced distance is used. These parameters vary only slightly with aspect ratio. If the actual distance instead of this reduced coordinate is used, an aspect ratio of one to ten would be required to obtain results which are accurate to 6%. These conclusions are not valid for a catalytic reactor with four catalytic walls.