We discuss the design of a Ni-Au nanoparticle catalyst system, which is based on the detailed experimental and theoretical understanding of the alloying and the chemical reaction processes on single-crystal surfaces. The alloy formation and structure of Ni-Au catalysts supported on SiO2 and on MgAl2O4 are simulated by Monte Carlo schemes as well as experimentally studied by a combination of in situ X-ray absorption fine structure, transmission electron microscopy, and in situ X-ray powder diffraction. On-line mass spectrometry is used to follow the reactivity of the catalyst and thermogravimetric analysis provided information on the deposition rate of carbon during steam reforming of n-butane. The simulations and the experiments give evidence for the formation of a Ni-Au surface alloy on the Ni particles fur both supports. The Ni-Au catalysts exhibiting the surface alloy are active for steam reforming and are more resistant toward carbon formation than the pure Ni catalyst. Blocking of highly reactive Ni edge and kink sites by Au atoms is presumably the reason for the increased robustness of the Ni-Au catalyst.