The growth of metal particles on ordered oxide surfaces provides a strategy to prepare well-defined model systems for supported catalysts, which can by easily studied by most surface-science techniques. Here, we focus on Palladium particles grown on an ordered Al2O3 film on NiAl(110), a system which has previously been characterized in detail with respect to its structural, electronic and adsorption properties. In this contribution, we will provide several examples, showing how adsorption and reactivity phenomena on these systems can be addressed over a pressure range from ultrahigh vacuum (UHV) to near atmospheric pressure. In the low pressure region, we apply a combination of molecular beam methods and in-situ infrared reflection absorption spectroscopy (IRAS). For CO adsorption, angular resolved scattering and sticking coefficient measurements and structural information allow us to quantify different adsorption channels including reverse spillover effects. The coverage dependent kinetics of CO oxidation is derived and discussed in comparison with the single crystal kinetics. The adsorption of CO on alumina supported Pd aggregates at low and high pressure, i.e. from 10(-7)-200 mbar, is examined by IR-VIS sum frequency generation (SFG) vibrational spectroscopy. At low pressure, the CO adsorption site distribution (bridged vs. on-top) depends on the particle surface structure and temperature, but under reaction conditions, the site occupancy is mainly governed by the CO pressure. The impact of these results on the extrapolation of UHV data to high pressure catalysis is discussed.