An instrument is introduced that combines a scanning tunneling microscope ͑STM͒ and a small volume ͑300 l͒ flow reactor, for the in situ study of catalytic surfaces at semi-industrial conditions. The tip is led through a gold membrane separating the reactor from the rest of the STM. We show ͑1͒ that the scanning through the membrane is possible with no noticeable effect of the membrane; ͑2͒ that it is possible to image surfaces in a gas flow of 10 ml/min, and that the imaging is unaffected by

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... derate changes in the flow conditions during the scanning; and ͑3͒ that imaging of atomic steps is possible on a Co͑0001͒ surface in a reactive environment at room temperature and at elevated temperatures. Furthermore, it is observed, that the step mobility on Co͑0001͒ is strongly enhanced in the presence of H 2 .