Molecular weight distribution has been measured in monodisperse polystyrene film (MW=9 000 amu) after ion bombardment, in the ion fluence range 1011–1013ions/cm2. The chosen beams are 100 keV He, 200 keV Ne, and 400 keV Ar. The experimental data have been interpreted in terms of a simple statistical model for cross‐links. The chemical yield is found to be very high and equal to 0.30, about a factor of 10 higher than the values given in the literature for gamma irradiation [M. Dole, inTheRadiationChemistryofMacromolecules(Academic, New York, 1973), Vol. 2, Chap. 5, p. 57].