Extensive experiments with a flow reactor were performed to reliably detect the steady-state catalytic formation of ethylene oxide from ethylene and oxygen on an Ag(111) single crystal. For single crystals with their small surfaces, this had been a major experimental problem because of the low reaction probability of ethylene and cross-sensitivities in the mass spectra. Using O-16(2) ethylene oxide could be detected on mass fragment m/z = 43, using O-18(2) on m/z = 31 and m/z = 45, but strong time-dependent background signals had to be subtracted. This was achieved by periodic temperature changes of the reactor. After proper inclusion of all effects, a mole fraction of ethylene oxide of only 3 x 10(-5) with respect to the reaction gas mixture, which was unequivocally produced by catalysis on the,Ag sample, could be determined with a signal-to-noise ratio of more than one order of magnitude above the detection limit. (C) 2012 Elsevier Inc. All rights reserved.