The adiabatic ionization energies and the threshold ion vibrational spectra of 3-methylindole and the 3-methylindole . H2O cluster and the hydrogen bonding energy of the latter have been measured with mass analyzed threshold ionization (MATI) technique. Dissociation of the cluster has been detected as a breakdown of the threshold ion signal at the parent mass channel and the simultaneous increase of the signal at the fragment mass channel. Comparison with our previous work on indole . H2O shows that there is only a small influence of the methyl group on the ionization energy and the hydrogen bonding strength. (C) 2004 Elsevier B.V. All rights reserved.