The reduced partition function ratios (RPFRs) of Li+(Solv)(n) (in which Solv = H2O, H2S, and CH3OH) clusters with different values were calculated, to investigate the solvent effect of the isotopic effect of the lithium. Structures of three solvated clusters-Li+(H2O)(n), Li+(H2S)(n), and Li+(CH3OH)(n)-were optimized by an ab initio molecular orbital method, and their RPFRs were calculated by frequency analysis. The RPFR of the solution was estimated by the extrapolation of the cluster values. The most-stable isomers of all three clusters for n greater than or equal to 4 have four solvent molecules in their first shell. The RPFR is dependent mainly on the number of solvent molecules in the first shell, and the size dependence of the RPFR plateaus at n = 4. The extrapolation of these values can be regarded as the RPFR in the solutions. The RPFRs are similar to1.07 for Li+(H2O)(n) and Li+(CH3OH), and are similar to1.03 for Li+(H2S)(n). The smaller RPFR of Li+(H2S)(n) is attributed to the smaller binding energy of the Li-S bond, which is weaker than that of the Li-O bond. The present results suggest the possibility of ionophores with S atoms (such as thioether, etc.) for lithium isotopic separation.