Abstract Water adsorption on the CaF 2 (111) cleavage plane was investigated by He-beam scattering after disturbing the surface with electron beams (with energies of 100, 300 and 500 eV) and ion beams. It is shown that different types of defects are produced, which sensitively influence both the He-diffraction intensities and the background scattering. Also, the adsorption behaviour of water on CaF 2 (111) is drastically changed. Water adsorption is observed even at room temperature due to electron induced defects. However, if the surface is sputtered with 500 eV Ne + ions, active defect sites are not produced and water adsorption does not occur up to 3 × 10 3 L.