Vibrational spectra of hetero-dimer and trimer ions containing aniline(+) are measured by infrared photodissociation spectroscopy. For the dimer ions, the NH2-bending overtone band gains its intensity through Fermi resonance interaction with the hydrogen-bonded NH stretching fundamental. Unperturbed frequencies of the NH2-bending overtone are calculated to be in the range of 3255-3276 cm(-1), suggesting that the frequency is almost intact upon cluster formation. For the trimer ions, Fermi resonance interaction occurs mainly between the NH2-bending overtone and the stretching fundamental of the NH oscillator involved in the stronger hydrogen bond. (C) 2003 Elsevier Science B.V. All rights reserved.