Photodissociation spectra of NH4+(NH3)n were measured for n=3 and 4 in the range of 1070–1680cm −1 by using an infrared free electron laser. Three vibrational bands observed for each species are assigned to the intramolecular vibrational modes: the ν 2 and ν 4 modes of NH 3 and the ν 4 mode of NH4+. When the first solvation shell is completed at n=4, the ν 2 (NH 3 ) and ν4(NH4+) wavenumbers are found to be higher by 182 and 36cm −1 , respectively, than those in free molecules, whereas the ν 4 (NH 3 ) wavenumber is lower by 29cm −1 .