The real structure of silver supported catalysts for ethylene epoxidation (Ag/alpha-Al2O3) was investigated using precision X-ray diffraction on synchrotron radiation, in situ high-temperature X-ray diffraction, and transmission electron microscopy. Stacking faults and intergrain boundaries were found to be main types of defects in the structure of silver microcrystals. In situ study revealed that under reaction conditions the intergrain boundaries remain unchanged, whereas stacking faults are annealed. We found a correlation between the reaction rate of ethylene epoxidation and parameters responsible for the relationship among the regular and defect regions in the bulk structure of silver particles.