The infrared spectra of the NH2 stretching vibrations of aniline(+)-Ar-n (n = 1-21) cluster cations have been measured by the depletion method. Aniline(+)-Ar-n clusters were prepared by near-resonant resonance-enhanced multiphoton ionization (REMPI) of larger aniline-Ar-m clusters. The red shifts of the NH stretching vibrations were 17-25 cm(-1), depending on the cluster size. The spectra of small clusters showed that there were hydrogen-bond-type (NH-Ar) and dispersion-type Ar-pi) isomers and that the NH-Ar isomer was more stable. The red shifts of the NH, stretching vibrations varied with the number of Ar atoms and showed a maximum at n = 12 and a local minimum at n approximate to 19. (C) 2002 Elsevier Science B.V. All rights reserved.