Predissocn. spectra were detd. for SF6-Ar clusters in a mol. beam by monitoring the decrease in a specific mass spectrometer signals induced by a tunable continuous-wave CO2 laser. A red shift occurs downstream from the nozzle as the gas expands and the clusters cool. Quite efficient isotope sepn. was obsd. due to vibrational predissocn. of van der Waals clusters in the SF6 beam seeded with Ar.