The efficiency of conventional aerodynamic isotope sepn. can be improved by 2 orders of magnitude with the aid of a relatively weak cw IR laser which is used to induce isotopically selective condensation. Overall isotope enrichment factors in excess of 2 are obtained as compared to about 1.02 in the conventional sepn. S isotopes in SF6 are sepd. on a lab. scale. IR vibrational predissocn. by itself and in combination with isotopically selective condensation are effective new ways of isotope sepn.