We investigated vibrational and electronic excitations of 0.1-layer up to 2.4-layer film of H2O deposited on a 1.4-layer film of thymine condensed on Ar at a temperature of 18 K using high-resolution electron-energy loss (EEL) spectroscopy at the incident energy of 12 eV. The spectral contribution originating essentially from the H2O overlayer is obtained by separating the measured contribution from the underlying film of thymine, considering the electron beam attenuation in the H2O overlayer. The vibrational EEL spectrum of submonolayer amount of H2O on thymine, which excepts for small energy shift of the vibrational bands, is found to compare in intensity to that of the same amount of H2O deposited directly on the argon. The electronic energy-loss intensity near 8.6 eV, which is attributed to the excitation of B-3,1(1) states of H2O in condensed phase, is observed to decrease by a factor of about 3 by the presence of the underlying film of thymine. This indicates that the corresponding cross section for excitation the B-3,1(1) states of H2O by the electron impact is reduced significantly by the close proximity of the thymine molecules. (C) 2004 American Institute of Physics.