3 Statistical evaluation of extended areas of atomic resolution STM images with chemical contrast can provide detailed, quantitative data on the abundance of different structural elements such as atomic ensembles on bimetallic surfaces, which provide a basis for the quantitative interpretation and modeling of spectroscopic results and kinetic results obtained on these surfaces. On a more qualitative scale, such information can also be obtained from spectroscopy measurements, e.g., via titration with suitable adsorbates.