The rotational relaxation of N-2 in collisions with He has been investigated in free jets with a combination of resonance-enhanced-multiphoton ionization spectroscopy and time-of-flight techniques. From the measured data, a global cross-section for rotational relaxation has been derived. The cross section grows from a value of approximate to8 Angstrom(2) at 100 K to approximate to18 Angstrom(2) at 5-15 K. For the weakest supersonic expansions investigated, a breakdown of the translational equilibrium between He and N2 has been observed, that could be well accounted for by an isentropic jet model and classical collision cross-sections. (C) 2002 Elsevier Science B.V. All rights reserved.