The possibility of selection of molecules embedded in superfluid helium nanodroplets (clusters) is studied. A method proposed is based on selective vibration excitation of molecules inside the droplets by intense IR laser radiation, that causes a considerable change in cluster size distribution in a beam, and a subsequent deflection of clusters of different size on different angles by crossing atomic beam. An analysis of the possibility of sulfur isotopes separation in the SF6 molecules inside the helium droplets is carried out. The results of analysis demonstrate that separation of molecules on their isotope (element) composition is possible. (C) 2004 Elsevier B.V. All rights reserved.