The prepeak structure of a 3 mol/kg solution,of LiCIC)(4) in propylene carlitinate (PC) was studied by both neutron diffraction with isotopic substitution (NDIS) and molecular dynamics (MD) simulation. The NDIS data showed that the intensity of the prepeak decreases experimentally with an increase in the scattering length of the lithium atom from Li-7 to Li-6 in PC-d(6). On the other hand, although the prepeak was observed in solutions of both PC-d(6) and PC-h(6), it disappears when the 1:1 mixture of PC-d(6) and PC-h(6) was used as the solvent. The prepeak structure and its variation with the isotope substitution were reproduced well by MD simulation, and they were explained in terms of the contrast of the scattering length densities of the ionic and nonpolar domains.