Ordered laminar flow and mixing driven mainly by molecular diffusion in microreactors enables operation using fluid segments. This paper discusses the effects of configuration of fluid segments including reactants at the reactor inlet for catalytic surface reactions in a microreactor. Irreversible and reversible parallel and series-parallel reaction systems were investigated. The rate-controlling step of these reaction systems is diffusion in the channel or diffusion in the catalyst layer. Under some diffusion-controlled conditions, appropriate fluid segment configuration gives higher selectivity of the desired product than reactors where the reactants are mixed at the reactor inlet. Configurations in which reactants with different reaction orders in parallel reactions or consumed only in the reaction producing the desired product are placed on the catalyst surface are suitable to improve the selectivity of the desired product. The configuration to maximize the yield of the desired product depends on the reaction system and rate-controlling step. To obtain a given selectivity of the desired product, the required fluid segment size depends on the configuration. The results reported here indicate an advantage of fluid segment, which is a typical fluid operation in microreactors, for enhancing the yield and selectivity of desired products. (C) 2007 Elsevier B.V. All rights reserved.