Probes of the collisional alignment of acetylene molecules, induced by seeding effects in supersonic expansion, are obtained by using both IR laser absorption and molecular beam scattering experiments. The dependence of the phenomenon on the rotational level and on the final molecular velocity is investigated. The effect of the pressure of the molecular beam source and of the angular cone aperture around the beam emerges also from the comparison of the experimental results. (c) 2007 Elsevier B.V. All rights reserved.