Efficient laser isotope sepn. is demonstrated in 2 distinct schemes. Irradn. of the collisional zone of the gas expansion provokes an isotopically selective condensation, as well as changes in the velocity distributions of the free jet. The resulting large changes in particle mass and/or changes in beam geometry, lead to sepn. of 32SF6, 33SF6 and 34SF6 in an SF6/Ar mixt. and enrichment factors in excess of 2.0. Br isotopes were sepd. in CF3Br/Ar mixts. and Si isotopes in SiF4/Ar mixts. Irradn. of the collision free part of the free jet induces single photon IR vibrational predissocn. of van der Waals clusters. Thus at selected wavelengths clusters contg. one isotopomer can be made to recoil out of the beam preferentially, resulting in the sepn. of isotopes. A tunable IR laser of only a few watts is used.