In highly-dispersed metal catalysts, the supporting oxide stabilises the metal particles, but may also promote individual reaction steps. In particular, many reactions are believed to proceed, preferentially or exclusively, at the metal-oxide interface which contains sites with distances between metal atoms and support in the range of atomic dimensions. In this paper, the physicochemical characterisation of these so-called adlineation sites and their possible catalytic effects are reviewed. It will be shown in various examples that they may have a particular activity and can be poisoned selectively. They may contribute to several particle size effects observed in hydrocarbon reactions, e.g. to ring opening and isomerization, but they may also be responsible for >C=O activation on noble metals supported by reducible oxides. Furthermore, they may play a role in selectively adsorbing chlorine which is an important component in catalyst restructuring. It will be attempted to detect and to relate common features of the observed phase boundary effects and to seek for a common explanation. The paper is, however, not intended as a review of strong metal-support interaction phenomena. The ring opening of methylcyclopentane on platinum is particularly sensitive to phase-boundary effects. Starting out from the authors’ results it will be shown how this reaction is influenced by modifications of the support, by changes of hydrogen pressure and reduction state, and by selectively poisoning the interface. Further on, the CO hydrogenation on supported Ph and the CH4-CO2 reforming reaction on Pt/ZrO2 serve as examples for >C=O activation, and the influence of Lewis acid sites at the boundary on the catalytic activity is discussed in more detail. The last chapter is dedicated to the role of adsorbed chlorine and to the partly controversial results concerning the effect of halogen additives on the properties of supported catalysts, including the possible existence of ’soluble platinum’.