For the surface alloys, the two-dimensional (2D) distribution of the surface atoms is a crucial parameter, as it determines the abundance of different types of adsorption sites. For AB surfaces with large islands of A or B surface areas ('2D phase separation'), the fraction of mixed AxBy type sites will be small, while for a random distribution this should be much larger, and it would be even larger for preferential mixing. The resulting 2D distributions (short-range order parameters (SROs) ), which were determined quantitatively from atomic resolution scanning tunneling microscopy (STM) images with chemical contrast for different surface concentrations of Ag and Pd or Ag and Pt , and trends therein with increasing Pd or Pt concentration provide a basis for the identification of preferential adsorption and reaction sites (see sections 3.1 and 3.2). Quantitative statistical evaluation of these measurements yielded an essentially random distribution of the Pd and Ag surface atoms for the different PdAg/Pd surfaces , while for PtAg/Pt systems the data revealed a clear tendency toward 2D phase separation . The impact on the adsorption properties will be discussed in section 3.1.