Low energy ion scattering provides a direct measurement of surface rippling at room temperature, i.e. a relaxation of Au atoms 0.12 Å above the Cu atoms in the outermost layer, which is unambiguously terminated by a mixed and ordered CuAu layer. It is shown that the technique, which is sensitive to the locations of atoms at the first monolayer, is also sensitive to the order - disorder transition in this layer. The occurrence of disorder modifies the arrangement of the atomic rows and then changes the double scattering conditions for low incident angles. As a result, the Au signal decreases significantly at the transition temperature. This effect is continuous and starts at about 50°C below the bulk transition temperature (663 K).