Comparative studies of supported metal catalysts are facilitated if the catalysts consist of well-faceted metal particles whose structural changes under reaction conditions are known. We describe the use of regular noble metal crystallites (Ir, Rh and Pt), obtained by epitaxial growth on a single crystal ( NaCl) substrate and subsequently contacted with the support by reactive high-vacuum deposition of the oxide, to study the inherent catalytic activity of the metals. Most of these metal particles exhibit defined zone axes parallel to the electron beam direction, which allows a characterization by selected area electron diffraction and lattice plane imaging in the electron microscope. The microstructural changes during activation processes ( oxidation and reduction at temperatures up to 723 K) have been studied and correlated with corresponding changes in the conversion of neopentane with excess hydrogen. Particular emphasis was placed on the dependence of the reaction rates on hydrogen pressure. The structural changes at the metal - oxide interface upon reduction above similar to 673 K may be interpreted as a prestage to alloy formation.