In order to understand the microsolvation of LiI and CsI in water and provide information about the dependence of solvation processes on different ions, we investigated the LiI(H2O)(n)(-) and CsI(H2O)(n)(-) (n = 0-6) clusters using photoelectron spectroscopy. The structures of these clusters and their corresponding neutrals were investigated with ab initio calculations and confirmed by comparing with the photoelectron spectroscopy experiments. Our studies show that the structural evolutions of LiI(H2O)(n) and CsI(H2O)(n) clusters are very different. The Li-I distance in LiI(H2O)(n)(-) increases abruptly at n = 3, whereas the abrupt elongation of the Li-I distance in neutral LiI(H2O)(n) occurs at n = 5. In contrast to the LiI(H2O)(n)(-) clusters, the Cs-I distance in CsI(H2O)(n)(-) increases significantly at n = 3, reaches a maximum at n = 4, and decreases again as n increases further. There is no abrupt change of the Cs-I distance in neutral CsI(H2O)(n) as n increases from 0 to 6. Water molecules interact strongly with the Li ion; consequently, water molecule(s) can insert within the Li+-I- ion pair. In contrast, five or six water molecules are not enough to induce obvious separation of the Cs+-I- ion pair since the Cs-water interaction is relatively weak compared to the Li-water interaction. Our work has shown that the structural variation and microsolvation in MI(H2O)(n) clusters are determined by the delicate balance between ion-ion, ion-water, and water-water interactions, which may have significant implications for the general understanding of salt effects in water solution.