Multiphase hydrogenation reactions involving solid catalyst play a critical role in the pharmaceutical industry. Nearly twenty percent of all reaction steps in a typical fine chemical synthesis are catalytic hydrogenation. Mass and heat transport resistances are generally an obstacle in such reaction systems. The microscale geometry of microreactor technology offers significant gains against these hurdles, hence their use would greatly benefit chemical processing in the pharmaceutical and other industries. A silicon microreactor has been developed to investigate multiphase mass transfer in the context of gas-liquid-solid catalytic reactions. The reactor employs a three-channel "catalyst-trap" design, whereby solid catalyst is suspended in the liquid channel by an arrangement of posts. Such a device supports the use of commercial catalyst, and allows control of pressure drop across the bed by engineering the packing density. This paper discusses the design and operation of the reactor, with the liquid-phase hydrogenation of o-nitroanisole to o-anisidine as a prototype reaction. Experiments are carried out across a range of gas and liquid flow rates that encompasses three distinct flow regimes, termed gas-dominated, liquid-dominated, and transitional. A two-phase "flow map" is generated independent of the reaction to identify the flow regime present at each set of conditions. Reaction experiments assign a conversion to each point in the flow map, in order to subsequently reconcile differences in performance with the characteristics of the respective flow regime. The highest reaction conversion occurs in the transitional flow regime, where competition between the two phases results in the generation of a large amount of gas-liquid interfacial area. Because the experimental conversion is greater than that predicted by the initial plug-flow model, we revise the model to account for the mass transfer enhancement induced by transitional flow. This reactor architecture may be useful for catalyst evaluation through rapid screening, or in large numbers as an alternative to macro-scale production reactors.