In order to gain information about the chemical reaction directly after the initial touch-down of a reactive species on a solvent thin film we have studied the reaction of amorphous water layers with sodium at 10 K substrate temperature. At this temperature a water layer can be described as a molecular solid and, therefore, bulk effects such as solvation of adsorbates and surface reaction products do not have to be taken into consideration. It is shown that, depending on the waterlayer thickness, incompletely shielded electronic state density of the supporting metal substrate can alter the reactivity of the adsorbed species. (C) 2003 Elsevier Science B.V. All rights reserved.