The term 'bimetallic surfaces' refers to surfaces, whose surface region -not necessarily only the topmost atomic layer -consists of two metal components whose physical and chemical properties are determined by these two components. This distinguishes them from so-called high entropy alloys, which according to their definition consist of five and more principal components with concentrations between 5 and 30% . Focusing on the distribution of the metal components in the surface region, this includes a number of different structural configurations. On the one hand, there are surfaces where the topmost layer(s) are intermixed, such as surfaces of metal bulk alloys or of two-dimensional (2D) alloy films of a monolayer (monolayer surface alloys) or a few layers thickness supported on a monometallic metal support . On the other hand, they include surfaces formed by an ultrathin film of one or more atomic layers thickness of metal A, which is deposited on a support formed by a different metal B, where the surface properties of the film are modified by the presence of the support.