Selective IR vibrational predissocn. (IRVP) of van der Waals clusters in a free jet is an effective way to sep. isotopes. Abs. sepn. factors a in excess of 1.1 were obsd. upon irradn. of a beam of SF6 dild. in Ar with a 20 W CO2 laser. Selection of the wavelength permits enrichment or depletion of any one of the S isotopes in the mixt. To understand the obsd. wavelength dependence of the isotope enrichment the effective IRVP spectra were measured of the individual van der Waals clusters (SF6)mArn with 1 ? m ? 3 and n ? 9. A systematic study of these spectra reveals a significant red-shift which occurs upon attaching Ar atoms to (SF6)m. Such shifts can attain as much as 10 cm-1, as in the case between SF6 and SF6.Ar9.