With decreasing size of crystals the number of their surface atoms becomes comparable to the number of bulk atoms and their powder diffraction pattern becomes sensitive to a changing surface structure. On the example of nanocrystalline gold supported on also nanocrystalline CeO2 we show evolution of (a) the background pattern due to chemisorption phenomena, (b) peak positions due to changing concentration profile of O within nonstoichiometric CeO(2−x) particles, (c) Au peak intensity due to transport phenomena of gold atoms to the surface of ceria, varying amount of Au in the crystalline form. The results of the measurements, complemented with mass spectrometry gas analysis, point to a multiply twinned structure of gold, its high mobility and repeatable peak position on exposure to He-X-He when X is O2 , H2 ,CO or CO oxidation reaction mixture, suggesting no ceria lattice oxygen being consumed/supplied at any stage of the process. The work shows possibility of structurally interpreting different contributions to the multi-phase powder diffraction pattern during a complex physico-chemical process, including effects of physi- and chemisorption. It shows a way to structurally interpret surface evolution during a catalytic reaction involving no bulk phase transition.