We have investigated the adsorption of cesium on GaAs(001) using synchrotron radiation excited photoemission spectroscopy and also by monitoring the optical transitions related to gallium dimers and arsenic dimers using reflectance anisotropy spectroscopy (RAS). Cesium adsorption has little effect on the Ga 3d core level shape. In sharp contrast, we observe modifications of the RA signal related to gallium dimers, induced by the modification of the electronic states involved in the corresponding surface optical transitions, The changes of the RA. spectrum reveal two adsorption phases at Cs coverages above and below 0.5 monolayers. The origin for the strong sensitivity of RAS to semiconductor/alkali metal interface formation is discussed.