The structure and the electronic valence state occupation of ultrathin K, Rb, and Cs films grown on a GaAs(1 0 0)-(4 x 2) surface have been studied by means of metastable He atom scattering (MHAS), He atom scattering (HAS), and low-energy electron diffraction (LEED) at temperatures ranging from 150 to 400 K. From the survival probability of the scattered He* atoms, detailed information on the coverage-dependent filling of the alkali metal valence states and their emptying upon subsequent exposure to oxygen were derived. These data reveal for K and Rb a nearly linear band filling with increasing coverage starting at about 0.5 ML whereas a more rapid filling is observed for Cs which is almost completed at about 0.7 ML Subsequent oxygen adsorption causes a demetallization of the metallic alkali metal monolayers. In case of Cs, a distinct minimum of the He* signal appears at an oxygen exposure of about 0.8 L, presumably indicating the onset of subsurface, oxidation.