Time-of-flight mass spectra have been measured for inner-shell ionization of ethanol molecules, its clusters, and methanol clusters at the oxygen K-edge regions. Only in the case of the ethanol clusters, a fairly intense signal corresponding to H3O+ has been observed. The measurements for ethanol-d(1) (C2H5OD) suggest that at least one H atom in the H3O+ ion originates from the C2H5 group of neighbouring ethanol. It is proposed that in addition to the O-H center dot center dot center dot OH type hydrogen bonding in the ethanol clusters, the H in the C2H5 group and OH interact with each other. (c) 2006 Elsevier B.V. All rights reserved.