New opportunities for narrowing the pressure and material gaps in studies of catalytic systems have been provided by the advances in synchrotron-radiation-based X-ray photoelectron spectroscopy (XPS). The recent achievements in this respect are illustrated by studies revealing the dynamics of the ruthenium oxidation state and the related to that activity and selectivity of the Ru catalyst in methanol oxidation reaction. The microscopy anisotropic oxide growth or oxide reduction, resulting in coexistence of transient RuOx and RuO2 states in a wide temperatureâ pressure range, evidenced by the XPS provided the basis for identifying the catalytically active states by in-situ high-pressure XPS studies of methanol oxidation. It has been shown that the nonequilibrium transient ruthenium oxides evolve under the specific reaction conditions of methanol oxidation and dramatic changes in the selectivity is correlated to relatively small differences in the oxygen content of the RuOx states, controlled by reactants partial pressures and temperatures.