The photoabsorption spectra of argon cluster ions, Ar-n(+) are calculated for n=4 to 25. The internal motion of the cluster is accounted for by the molecular dynamics method. The diatomicsin-molecules (DIM) potential energy surface is used for the calculation. There are basically two peaks in the spectra. At a low internal energy, the primary peak shifts from 510 to 550 nn at n similar or equal to 10, and then shifts abruptly to 600 nm at n similar or equal to 14. As the internal energy increases, the spectra become broad and the clear transition disappears. The spectral shift is explained by the solvation of the ion core in the cluster, with the rest of the constituent atoms acting as solvent atoms. The first red shift is due to the crossover of the energy levels between the ion core and the solvent shell. The second one takes place because the solvation energy is increased abruptly, which is explained in terms of the structural change in the solvation shell.