Relaxation and alignment of the rotational angular momentum of ethylene molecules are here shown to be induced naturally by collisions occurring in supersonic seeded expansions, which lead to generation of continuous molecular beams. Dependenes of the achieved alignment degree on the rotational level, on the final molecular speed and on the divergence angle with respect to the molecular beam expansion direction have been investigated using two separate complementary experimental techniques, IR spectroscopy and molecular beam scattering. Results obtained for this planar hydrocarbon are of interest toward the goal of planning collisional alignment experiments on molecules with different mass and geometry. (c) 2005 Elsevier B.V. All rights reserved.