We have bombarded amorphous water clusters adsorbed on Cu(1 1 1) with electrons from the tip of a low-temperature scanning tunnelling microscope. Dissociation of the water molecules is induced by reaching an anti-bonding orbital at U-D = (1.6 +/- 0.5) V. For voltages below U-D and above U-o approximate to 3 V our measurements indicate crystallisation of the molecular clusters and oxidation of the underlying substrate, respectively. (C) 2002 Published by Elsevier Science B.V.