Vibrational predissociation spectra of NH4 +(NH3) n -1 (n = 6−9) were measured in the wavenumbers of 1045−1091 cm-1 by using a tandem mass spectrometer equipped with a serpentine octopole ion guide and a line-tunable cw CO2 laser. Each spectrum consists of an intense and broad peak and a weak one. The dissociation cross sections were in the order of 10-17−10-18 cm2. As n increases, the wavenumber of the intense and broad peak red-shifts gradually, and its intensity increases. The Hartree−Fock and the second-order Møller−Plesset calculations were performed to obtain the wavenumbers and the intensities of the optical absorption by these cluster ions with n = 2−8. The comparison of the experimental results with the calculations leads us to conclude that NH4 +(NH3) n -1 forms a shell structure and causes the absorption band associated with the collective ν2 vibration of NH3 to split.