An automated reactor system for a detailed performance evaluation of gas-phase heterogeneous oxidation catalysts that utilizes a parallel array of six fixed microreactors called the Multiple Automated Reactor System, or MARS, is described. The key MARS components include a gas manifold that safely generates a light hydrocarbon oxidation feed composition, an array of six fixed-bed microreactors with dedicated components for control of individual reactor feed gas flow rates and temperatures, an integrated gas sampling and gas chromatography system for online analysis of feed and product gas compositions, and a process automation control package based on process logic controller technology. The addition of one or more liquid feed components, such as steam or organometallic catalyst surface modification agents, is also possible through a dedicated liquid feed vaporizer subsystem. The automation package contains all of the elements needed for logging of process sensors, monitoring of all process alarms, control of all process variables, interlock sequencing, and communication between the operator and automation hardware through a human-machine interface. These features allow a user-defined catalyst testing protocol to be downloaded from the automation so that the system can safely operate 24/7 in an unattended mode. Two versions of the MARS are described that mainly differ in the fixed-bed microreactor configuration and the length of the heated zones used for transport of product gases from the catalytic zone to the online gas sampling system. One version employs a classical U-tube fixed-bed microreactor, whereas the second version uses a straight-through fixed bed. An overview of key operating characteristics is provided. It is shown in part 2 of this series of papers that the contribution of metal-wall-catalyzed and homogeneous gas-phase reactions on the observed hydrocarbon and oxygen conversions can achieve significant yet different levels in each reactor configuration for 1,3-butadiene oxidation. The need to first assess the role of undesired reactions and then to either eliminate or minimize their contribution to the desired solid-catalyzed reaction in parallel microreactors is emphasized.