[en] The far infrared absorption spectrum of the lithium perchlorate/benzo-15-crown-5 complex separated from various solvent has been investigated in the 250∼590 cm^-1 region. It has been found that the change in the medium used for the preparation of the complex probably leads to the change in the far infrared shift. A linear function of far infrared absorption band versus lithium isotope abundance in the complex was discovered. The far infrared absorption band of the complex containing 100% ⁶Li could be determined by extrapolation. A concept of relative reduced partition function ratio was proposed for the first time in order to calculate approximately the isotope effect of lithium by use of the data of the far infrared spectra. (author)