This paper deals with reviewing on the application of micro-structured reactors for heterogeneously catalysed gas phase reactions. After a brief introduction covering some estimation criteria for the performance of micro-structured reactors, an overview of the work performed to date in the field is given. The reactors are classified by the type of catalyst applied (porous or non-porous) and according to their basic design criteria. At the end, a small chapter is dedicated to applications employing reactors combined with other micro-structured devices like heat exchangers. Finally, some alternative ways of achieving micro-structures, besides relying on micro-fabrication, are discussed. Diverse gas-phase reactions have been investigated in micro-reactors, among them (partial) oxidations, hydrogenations, dehydrogenations, dehydrations, and reforming processes. Particular attention has been drawn to achieve excellent temperature control and to prevent hot-spots. So. for many reactions increases in selectivity were found. Especially, many examples of partial oxidations were described, including processes of utmost industrial importance such as ethylene oxide synthesis. Within consecutive processes, as e.g. given for multiple hydrogenations, high selectivity was achieved for species that are thermodynamically not the most stable molecule of all species serially generated such as monoenes yielded by hydrogenation of polyenes. Also, increases in conversion were achieved, e.g. by processing at high pressure and high temperature, often in the explosive regime. As a consequence, high space-time yields were reported as well. In many cases, reactor performance better compared to fixed-bed technology was achieved. Process safety was found to be high when using micro-reaction devices; intrinsic safety in former explosive regimes was ascribed. With respect to process optimisation, fast serial screening of process parameter variation was conducted, at low sample consumption. (C) 2003 Elsevier B.V. All rights reserved.