STM imaging also provided detailed information on the nature and abundance of the structural defects created during preparation of the bimetallic model surfaces, which is mostly done by vapor deposition of the respective metal(s) and subsequent annealing. In many if not most cases, the resulting surface morphology is determined by kinetic limitations rather than by the thermodynamic system properties (see section 2.1). Depending on the deposition and subsequent annealing temperatures, this may result in highly defective surfaces, with large numbers of different island structures, if the temperature is too low to reach the thermodynamically expected smooth form. On the other hand, too high temperatures may result in the onset of surface intermixing, which is not easily detectable or even quantifiable with standard surface techniques. Furthermore, also adsorption or reaction may result in a restructuring of the bimetallic surface, e.g., in a partial de-alloying of the surface layer due to segregation / bulk dissolution or, in electrochemistry, due to electrochemical dissolution . Because of the considerable influence of such defects on the adsorption and in particular on the reaction properties, the correct interpretation of the results of adsorption and/or reaction measurements on bimetallic surfaces requires a careful check of the defect structure of the respective surface and changes therein. This is most directly achieved by comparison of representative STM images taken before and after the adsorption / desorption / reaction process.