Collision energy dependence of partial ionization cross-sections of C2H2 by collision with He*(2(3)S) metastable atoms in low collision energy regions (20-50 meV) was first observed using a liquid N-2 cooled discharge nozzle source. Negative collision energy dependence was found for ionizations from sigma(CH) molecular orbitals, while positive collision energy dependence has been obtained in higher collision energy region of 100-360 meV. These results can be explained by the anisotropic interaction of the present system, and especially the low collision energy feature should indicate that a shallow attractive well exists around hydrogen atom. (C) 2004 Elsevier B.V. All rights reserved.