A mol. beam mass spectrometer was constructed to study the formation of van der Waals clusters over a wide mass range. Using this app. (SF6)m was detected with 1 ? m ? 100 in the adiabatic expansion of SF6. By irradiating the SF6 jet with a continuous-wave IR laser beam in the collisional region of the expansion with power densities as low as .apprx.100 W cm-2, cluster formation can be essentially completely inhibited due to a shift in the \"equil.\" (SF6)m + SF6 = (SF6)m-1 towards the reactants, caused by the laser-induced excitation of the SF6 monomers. Irradiating the mol. beam downstream in the collision free region induces a smaller decrease in the cluster concns. The latter effect is interpreted as being due to single quantum IR induced predissocn. of the weakly bound van der Waals mols. The power d. and wavelength dependence of these effects are reported and discussed.